RESUMEN
Relay cells of dorsal lateral geniculate nucleus (LGN) receive a Class 1 glutamatergic input from the retina and a Class 2 input from cortical layer 6. Among the properties of Class 2 synapses is the ability to activate metabotropic glutamate receptors (mGluRs), and mGluR activation is known to affect thalamocortical transmission via regulating retinogeniculate and thalamocortical synapses. Using brain slices, we studied the effects of Group I (dihydroxyphenylglycine) and Group II ((2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine) mGluR agonists on retinogeniculate synapses. We showed that both agonists inhibit retinogeniculate excitatory postsynaptic currents (EPSCs) through presynaptic mechanisms, and their effects are additive and independent. We also found high-frequency stimulation of the layer 6 corticothalamic input produced a similar suppression of retinogeniculate EPSCs, suggesting layer 6 projection to LGN as a plausible source of activating these presynaptic mGluRs.
Asunto(s)
Cuerpos Geniculados/fisiología , Inhibición Neural/fisiología , Receptores de Glutamato Metabotrópico/fisiología , Retina/fisiología , Transmisión Sináptica/fisiología , Animales , Corteza Cerebral/fisiología , Ciclopropanos/farmacología , Agonistas de Aminoácidos Excitadores/farmacología , Potenciales Postsinápticos Excitadores/efectos de los fármacos , Potenciales Postsinápticos Excitadores/fisiología , Cuerpos Geniculados/efectos de los fármacos , Glicina/análogos & derivados , Glicina/farmacología , Ratones , Vías Nerviosas/efectos de los fármacos , Vías Nerviosas/fisiología , Estimulación Luminosa , Receptores de Glutamato Metabotrópico/agonistas , Resorcinoles/farmacología , Retina/efectos de los fármacos , Transmisión Sináptica/efectos de los fármacos , Tálamo/fisiología , Vías Visuales/fisiologíaRESUMEN
We investigated the use of flavoprotein autofluorescence (FA) as a tool to map long-range neural connections and combined FA with laser-uncaging of glutamate to facilitate rapid long-range mapping in vitro. Using the somatosensory thalamocortical slice, we determined that the spatial resolution of FA is >or=100-200 microm and that the sensitivity for detecting thalamocortical synaptic activity approximates that of whole cell recording. Blockade of ionotropic glutamate receptors with DNQX and AP5 abolished cortical responses to electrical thalamic stimulation. The combination of FA with photostimulation using caged glutamate revealed robust long-distance connectivity patterns that could be readily assessed in slices from the somatosensory, auditory, and visual systems that contained thalamocortical, corticothalamic, or corticocortical connections. We mapped the projection from the ventral posterior nucleus of thalamus (VPM) to the primary somatosensory cortex-barrel field and confirmed topography that had been previously described using more laborious methods. We also produced a novel map of the projections from the VPM to the thalamic reticular nucleus, showing precise topography along the dorsoventral axis. Importantly, only about 30 s were needed to generate the connectivity map (six stimulus locations). These data suggest that FA is a sensitive tool for exploring and measuring connectivity and, when coupled with glutamate photostimulation, can rapidly map long-range projections in a single animal.
Asunto(s)
Mapeo Encefálico , Flavoproteínas/metabolismo , Rayos Láser , Vías Nerviosas/fisiología , Estimulación Luminosa/métodos , Sinapsis/fisiología , 6-Ciano 7-nitroquinoxalina 2,3-diona/farmacología , Animales , Animales Recién Nacidos , Antagonistas de Aminoácidos Excitadores/farmacología , Femenino , Análisis de Fourier , Glutamatos/farmacología , Técnicas In Vitro , Masculino , Potenciales de la Membrana/efectos de los fármacos , Potenciales de la Membrana/fisiología , Ratones , Ratones Endogámicos BALB C , Técnicas de Placa-Clamp/métodos , Ratas , Ratas Sprague-Dawley , Corteza Somatosensorial/fisiología , Sinapsis/efectos de los fármacos , Tálamo/fisiología , Factores de Tiempo , Valina/análogos & derivados , Valina/farmacologíaRESUMEN
We used electron microscopy to determine the relative numbers of the three synaptic terminal types, RL (round vesicle, large terminal), RS (round vesicles, small terminal), and F (flattened vesicles), found in several representative thalamic nuclei in cats chosen as representative examples of first and higher order thalamic nuclei, where the first order nuclei relay subcortical information mainly to primary sensory cortex, and the higher order nuclei largely relay information from one cortical area to another. The nuclei sampled were the first order ventral posterior nucleus (somatosensory) and the ventral portion of the medial geniculate nucleus (auditory), and the higher order posterior nucleus (somatosensory) and the medial portion of the medial geniculate nucleus (auditory). We found that the relative percentage of synapses from RL terminals varied significantly among these nuclei, these values being higher for first order nuclei (12.6% for the ventral posterior nucleus and 8.2% for the ventral portion of the medial geniculate nucleus) than for the higher order nuclei (5.4% for the posterior nucleus, and 3.5% for the medial portion of the medial geniculate nucleus). This is consistent with a similar analysis of first and higher order nuclei for the visual system (the lateral geniculate nucleus and pulvinar, respectively). Since synapses from RL terminals represent the main information to be relayed, whereas synapses from F and RS terminals are modulatory in function, we conclude that there is relatively more modulation of the thalamic relay in the cortico-thalamo-cortical higher order pathway than in first order relays.
Asunto(s)
Vías Nerviosas/anatomía & histología , Neuronas/ultraestructura , Sinapsis/fisiología , Tálamo/citología , Animales , Gatos , Microscopía Electrónica de Transmisión/métodos , Microscopía Inmunoelectrónica/métodos , Neuronas/metabolismo , Sinapsis/ultraestructura , Ácido gamma-Aminobutírico/metabolismoRESUMEN
Many of the ascending pathways to the thalamus have branches involved in movement control. In addition, the recently defined, rich innervation of 'higher' thalamic nuclei (such as the pulvinar) from pyramidal cells in layer five of the neocortex also comes from branches of long descending axons that supply motor structures. For many higher thalamic nuclei the clue to understanding the messages that are relayed to the cortex will depend on knowing the nature of these layer five motor outputs and on defining how messages from groups of functionally distinct output types are combined as inputs to higher cortical areas. Current evidence indicates that many and possibly all thalamic relays to the neocortex are about instructions that cortical and subcortical neurons are contributing to movement control. The perceptual functions of the cortex can thus be seen to represent abstractions from ongoing motor instructions.
Asunto(s)
Tálamo/fisiología , Vías Aferentes/fisiología , Animales , Tronco Encefálico/fisiología , Gatos , Corteza Cerebral/fisiología , Vías Eferentes/fisiología , Modelos Neurológicos , Médula Espinal/fisiología , Vías Visuales/fisiologíaRESUMEN
The lateral geniculate nucleus is the best understood thalamic relay. Only 5-10% of the inputs to geniculate relay cells derive from retina, which is the driving input. The rest, being modulatory, derive from local inhibitory inputs, descending inputs from visual cortex, and ascending inputs from brainstem. The nonretinal, modulatory inputs, which form the vast majority, dynamically control the nature of the geniculate relay. Among other actions, these modulatory inputs regulate membrane properties of relay cells and thereby control their mode of response to retinal inputs, and this dramatically affects the nature of information relayed to cortex. Our studies of the lateral geniculate nucleus of the cat lead to the speculation that this dynamic control depends on the animal's behavioral state and represents the neuronal substrate for many forms of visual attention. The lateral geniculate nucleus is a first-order relay, because it relays subcortical (i.e. retinal) information to cortex for the first time. In contrast, the other main thalamic relay of visual information, the pulvinar (and lateral posterior nucleus in carnivores), is largely a higher-order relay, since much of it seems to relay information from one cortical area to another. Much more corticocortical processing may involve these 're-entry' routes than has been hitherto appreciated. If so, the thalamus sits at an indispensable position for corticocortical processing.
Asunto(s)
Tálamo/fisiología , Vías Visuales/fisiología , Potenciales de Acción/fisiología , Animales , Calcio/fisiología , Cuerpos Geniculados/fisiología , HumanosRESUMEN
The low-threshold spike (LTS), generated by the transient Ca(2+) current I(T), plays a pivotal role in thalamic relay cell responsiveness and thus in the nature of the thalamic relay. By injecting depolarizing current ramps at various rates to manipulate the slope of membrane depolarization (dV/dt), we found that an LTS occurred only if dV/dt exceeded a minimum value of approximately 5-12 mV/sec. We injected current ramps of variable dV/dt into relay cells that were sufficiently hyperpolarized to de-inactivate I(T) completely. Higher values of dV/dt activated an LTS. However, lower values of dV/dt eventually led to tonic firing without ever activating an LTS; apparently, the inactivation of I(T) proceeded before I(T) could be recruited. Because the maximum rate of rise of the LTS decreased with slower activating ramps of injected current, we conclude that slower ramps allow increasing inactivation of I(T) before the threshold for its activation gating is reached, and when the injected ramps have a sufficiently low dV/dt, the inactivation is severe enough to prevent activation of an LTS. In the presence of Cs(+), we found that even the lowest dV/dt that we applied led to LTS activation, apparently because Cs(+) reduced the K(+) "leak" conductance and increased neuronal input resistance. Nonetheless, under normal conditions, our data suggest that there is neither significant window current (related to the overlap of the inactivation and activation curves for I(T)), rhythmogenic properties, nor bistability properties for these neurons. Our theoretical results using a minimal model of LTS excitability in these neurons are consistent with the experimental observations and support our conclusions. We suggest that inputs activating very slow EPSPs (i.e., via metabotropic receptors) may be able to inactivate I(T) without generating sizable I(T) and a spurious burst of action potentials to cortex.
Asunto(s)
Potenciales de Acción/fisiología , Cuerpos Geniculados/fisiología , Neuronas/fisiología , Potenciales de Acción/efectos de los fármacos , Animales , Señalización del Calcio/fisiología , Gatos , Cesio/farmacología , Simulación por Computador , Estimulación Eléctrica/métodos , Potenciales Postsinápticos Excitadores/efectos de los fármacos , Potenciales Postsinápticos Excitadores/fisiología , Cuerpos Geniculados/citología , Técnicas In Vitro , Lisina/análogos & derivados , Modelos Neurológicos , Neuronas/citología , Neuronas/efectos de los fármacos , Potasio/metabolismo , Tiempo de Reacción/efectos de los fármacos , Tiempo de Reacción/fisiología , Umbral Sensorial/fisiología , Tetrodotoxina/farmacología , Tálamo/citología , Tálamo/fisiologíaRESUMEN
All thalamic relay cells exhibit two distinct response modes--tonic and burst--that reflect the status of a voltage-dependent, intrinsic membrane conductance. Both response modes efficiently relay information to the cortex in behaving animals, but have markedly different consequences for information processing. The lateral geniculate nucleus, which is the thalamic relay of retinal information to cortex, provides a reasonable model for all of thalamus. Compared with burst mode, geniculate relay cells that are firing in tonic mode exhibit better linear summation, but have poorer detectability for visual stimuli. The switch between the response modes can be controlled by nonretinal, modulatory afferents to these cells, such as the feedback pathway from cortex. This allows the thalamus to provide a dynamic relay that affects the nature and format of information that reaches the cortex.
Asunto(s)
Potenciales de Acción/fisiología , Corteza Cerebral/fisiología , Potenciales Postsinápticos Excitadores/fisiología , Red Nerviosa/fisiología , Sinapsis/fisiología , Tálamo/fisiología , Animales , Humanos , Corteza Visual/fisiologíaRESUMEN
We show for the first time with in vitro recording that burst firing in thalamic relay cells of the monkey is evoked by activation of voltage-dependent, low threshold Ca(2+) spikes (LTSs), as has been described in other mammals. Due to variations in LTS amplitude, the number of action potentials evoked by an LTS could vary between 1 and 8. These data confirm the presence of two modes of firing in the monkey for thalamic relay cells, tonic and burst, the latter related to the activation of LTSs. With these details of the cellular processes underlying burst firing, we could account for many of the firing patterns we recorded from the lateral geniculate nucleus of the thalamus in behaving monkeys. In particular, we found clear evidence of burst firing during alert wakefulness, which had been thought to occur only during sleep or certain pathological states. This makes it likely that the burst firing seen in awake humans has the same cellular basis of LTSs, and this supports previous suggestions that burst firing represents an important relay mode for visual processing.
Asunto(s)
Conducta Animal/fisiología , Neuronas/fisiología , Tálamo/fisiología , Visión Ocular/fisiología , Potenciales de Acción/fisiología , Animales , Nivel de Alerta/fisiología , Calcio/fisiología , Umbral Diferencial , Electrofisiología , Cuerpos Geniculados/citología , Cuerpos Geniculados/fisiología , Técnicas In Vitro , Macaca fascicularis , Macaca mulatta , Tálamo/citología , Vigilia/fisiologíaAsunto(s)
Cuerpos Geniculados/fisiología , Neuronas/fisiología , Orientación/fisiología , Tálamo/fisiología , Corteza Visual/fisiología , Animales , Hurones , Aprendizaje/fisiología , Reconocimiento Visual de Modelos/fisiología , Retina/fisiología , Transmisión Sináptica/fisiología , Vías Visuales/fisiologíaRESUMEN
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.
Asunto(s)
Calcio/fisiología , Cuerpos Geniculados/fisiología , Modelos Neurológicos , Potenciales de Acción/fisiología , Animales , Gatos , Simulación por Computador , Umbral Diferencial/fisiología , Femenino , Cuerpos Geniculados/citología , Masculino , Neuronas/fisiología , Técnicas de Placa-Clamp , Tiempo de Reacción/fisiología , Tálamo/citología , Tálamo/fisiologíaRESUMEN
The relay of information through thalamus to cortex is dynamically gated, as illustrated by the retinogeniculocortical pathway. Important to this is the inhibitory interneuron in the lateral geniculate nucleus (LGN). For the typical neuron, synaptic information arrives through postsynaptic dendrites and is transmitted by axon terminals. However, the typical thalamic interneuron, in addition to conventional axonal outputs, has distal dendrites that serve both pre- and postsynaptic roles. These dendritic terminals participate in curious and enigmatic triadic arrangements, in which each contacts a relay cell dendrite and is contacted by a glutamatergic retinal terminal that innervates the same relay cell dendrite. Here we show that agonists of the metabotropic glutamate receptor (mGluR) activate dendritic terminals of interneurons in the absence of action potentials, thereby inhibiting the postsynaptic relay neuron. Somatic recordings from LGN interneurons reveal that there is no response to mGluR agonists, suggesting that their dendritic terminals are electrically isolated from their somata and axons, consistent with anatomical modelling of these cells. Our results offer insight into the functioning of triadic circuitry and indicate that thalamic interneurons can perform independent computations expressed through axonal as opposed to dendritic outputs.
Asunto(s)
Dendritas/fisiología , Ácido Glutámico/fisiología , Interneuronas/fisiología , Tálamo/fisiología , Animales , Gatos , Cicloleucina/análogos & derivados , Cuerpos Geniculados/citología , Cuerpos Geniculados/fisiología , Técnicas In Vitro , Inhibición Neural , Técnicas de Placa-Clamp , Ratas , Receptores de Glutamato Metabotrópico/agonistas , Receptores de Glutamato Metabotrópico/fisiología , Tetrodotoxina/farmacología , Tálamo/citologíaRESUMEN
When one nerve cell acts on another, its postsynaptic effect can vary greatly. In sensory systems, inputs from "drivers" can be differentiated from those of "modulators." The driver can be identified as the transmitter of receptive field properties; the modulator can be identified as altering the probability of certain aspects of that transmission. Where receptive fields are not available, the distinction is more difficult and currently is undefined. We use the visual pathways, particularly the thalamic geniculate relay for which much relevant evidence is available, to explore ways in which drivers can be distinguished from modulators. The extent to which the distinction may apply first to other parts of the thalamus and then, possibly, to other parts of the brain is considered. We suggest the following distinctions: Cross-correlograms from driver inputs have sharper peaks than those from modulators; there are likely to be few drivers but many modulators for any one cell; and drivers are likely to act only through ionotropic receptors having a fast postsynaptic effect whereas modulators also are likely to activate metabotropic receptors having a slow and prolonged postsynaptic effect.
Asunto(s)
Plasticidad Neuronal/fisiología , Neuronas/fisiología , Tálamo/fisiología , Animales , Transmisión Sináptica/fisiologíaRESUMEN
The thalamus has long been seen as responsible for relaying information on the way to the cerebral cortex, but it has not been until the last decade or so that the functional nature of this relay has attracted significant attention. Whereas earlier views tended to relegate thalamic function to a simple, machine-like relay process, recent research, reviewed in this article, demonstrates complicated circuitry and a rich array of membrane properties underlying the thalamic relay. It is now clear that the thalamic relay does not have merely a trivial function. Suggestions that the thalamic circuits and cell properties only come into play during certain phases of sleep to effectively disconnect the relay are correct as far as they go, but they are incomplete, because they fail to take into account interesting and variable properties of the relay that, we argue, occur during normal waking behavior. Although the specific function of the circuits and cellular properties of the thalamic relay for waking behavior is far from clear, we offer two related hypotheses based on recent experimental evidence. One is that the thalamus is not used just to relay peripheral information from, for example, visual, auditory, or cerebellar inputs, but that some thalamic nuclei are arranged instead to relay information from one cortical area to another. The second is that the thalamus is not a simple, passive relay of information to cortex but instead is involved in many dynamic processes that significantly alter the nature of the information relayed to cortex.
Asunto(s)
Corteza Cerebral/fisiología , Tálamo/fisiología , Vías Aferentes/anatomía & histología , Vías Aferentes/citología , Vías Aferentes/fisiología , Animales , Corteza Cerebral/anatomía & histología , Corteza Cerebral/citología , Humanos , Vías Nerviosas/anatomía & histología , Vías Nerviosas/citología , Vías Nerviosas/fisiología , Neuronas Aferentes/fisiología , Tálamo/anatomía & histología , Tálamo/citologíaRESUMEN
Prior morphological studies of individual retinal X and Y axon arbors based on intraaxonal labeling with horseradish peroxidase have been limited by restricted diffusion or transport of the label. We used biocytin instead as the intraaxonal label, and this completely delineated each of our six X and 14 Y axons, including both thalamic and midbrain arbors. Arbors in the lateral geniculate nucleus appeared generally as has been well documented previously. Interestingly, all of the labeled axons projected a branch beyond thalamus to the midbrain. Each X axon formed a terminal arbor in the pretectum, but none continued to the superior colliculus. In contrast, 11 of 14 Y axons innervated both the pretectum and the superior colliculus, one innervated only the pretectum, and two innervated only the superior colliculus. Two of the Y axons were quite unusual in that their receptive fields were located well into the hemifield ipsilateral with respect to the hemisphere into which they were injected. These axons exhibited remarkable arbors in the lateral geniculate nucleus, diffusely innervating the C-laminae and medial interlaminar nucleus, but, unlike all other X and Y arbors, they did not innervate the A-laminae at all. In addition to these qualitative observations, we analyzed a number of quantitative features of these axons in terms of numbers and distributions of terminal boutons. We found that Y arbors contained more boutons than did X arbors in both thalamus and midbrain. Also, for axons with receptive fields in the contralateral hemifield (all X and all but two Y axons), 90-95% of their boutons terminated in the lateral geniculate nucleus; the other two Y axons had more of their arbors located in midbrain.
Asunto(s)
Axones/química , Gatos/anatomía & histología , Mesencéfalo/anatomía & histología , Retina/ultraestructura , Tálamo/anatomía & histología , Animales , Gatos/metabolismo , Vías Eferentes/química , Vías Eferentes/ultraestructura , Peroxidasa de Rábano Silvestre , Lisina/análogos & derivados , Mesencéfalo/química , Microinyecciones , Terminaciones Nerviosas/química , Terminaciones Nerviosas/ultraestructura , Retina/química , Colículos Superiores/química , Colículos Superiores/ultraestructura , Tálamo/químicaRESUMEN
We constructed average histograms from responses evoked by flashing stimuli and noted previously described variations in the shape of the response profile, particularly with respect to sharpness of the peak. To express this variable, we measured the half-rise latency, which is the latency from stimulus onset required to reach half the maximum response. A short half-rise latency, which is characteristic of nonlagged cells, is associated with a brisk response and sharp peak; a long half-rise latency, characteristic of lagged cells, is associated with a sluggish response and broad peak. Nonlagged cells were readily seen; we attempted to identify cells with long latencies as lagged, but we were unable to do so unambiguously due to failure to observe lagged properties other than latency. We thus refer to these latter cells as having "lagged-like" responses to indicate that we are not certain whether these are indeed lagged cells. In addition to the histograms, we analyzed the individual response trials that were summed to create each histogram, and we used spike density analysis to estimate the initial response latency to the flashing spot for each trial. We found that lagged-like responses were associated with more variability in initial response latency than were nonlagged responses. We then employed an alignment procedure to eliminate latency variation from individual trials; that is, responses during individual trials were shifted in time as needed so that each had a latency equal to the average latency of all trials. We used these "aligned" trials to create a second, "aligned" response histogram for each cell. The alignment procedure had little effect on nonlagged responses, because these were already well aligned due to consistent response latencies amongst trials. For lagged-like responses, however, the alignment made a dramatic difference. The aligned histograms looked very much like those for nonlagged responses: the responses appeared brisk, with a sharply rising peak that was fairly high in amplitude. We thus conclude that the slow build up to a relatively low peak of firing of the lagged-like response histogram is not an accurate reflection of responses on single trials. Instead, the sluggishness of lagged-like responses inferred from average response histograms results from temporal smearing due to latency variability amongst trials. We thus conclude that there is relatively little difference in briskness between nonlagged and lagged-like responses to single stimuli.
Asunto(s)
Estimulación Luminosa , Tiempo de Reacción , Tálamo/fisiología , Animales , Gatos , Electrofisiología , Neuronas/fisiología , Factores de TiempoRESUMEN
Transmission through the lateral geniculate nucleus is facilitated following activation of the cholinergic input from the brain stem, which is thought to reflect activity patterns seen during arousal. One of the underlying mechanisms is the suppression of inhibitory circuits local to the lateral geniculate nucleus. However, evidence exists that some visually driven inhibitory inputs to geniculate nucleus. preserved or even enhanced under conditions of arousal, and during electrical activation of the parabrachial region of the brain stem. We have therefore reexamined the effect of brain-stem activation on the visual responses of one group of local inhibitory inputs to geniculate relay cells, those emanating from the adjacent perigeniculate nucleus. We recorded single perigeniculate cells in anesthetized, paralyzed cats. Axons innervating the lateral geniculate and perigeniculate nuclei from the parabrachial region of the brain stem were electrically activated, and the effect of this activation was assessed on both spontaneous and visually evoked responses. Visual stimulation consisted of sinusoidally modulated sine-wave gratings of varying spatial and temporal frequency. For the great majority of perigeniculate cells (32 of 40), brain-stem activation inhibited spontaneous activity, while one cell was excited, three showed a mixed effect and four were unaffected. Nevertheless, the responses of most cells (30 of 40) were facilitated when brain-stem activation was paired with certain spatio-temporal patterns of visual stimulation. Spatial tuning curves were constructed for 17 cells and temporal tuning curves for 14, before and during parabrachial activation. The responses of any one cell could be facilitated, unchanged, or suppressed, depending on the visual stimulus used. In some cases, this substantially modified the cell's spatial and temporal tuning properties. We conclude that activation of the brain stem disinhibits geniculate relay cells in the absence of visual stimulation, but it has the potential to enhance either the magnitude or specificity of visually driven inhibition arising from the perigeniculate nucleus.
Asunto(s)
Tronco Encefálico/fisiología , Potenciales Evocados Visuales/fisiología , Cuerpos Geniculados/fisiología , Neuronas/fisiología , Animales , Gatos , Estimulación Eléctrica , Retina/fisiología , Tálamo/fisiología , Vías Visuales/fisiologíaRESUMEN
1. Thalamic relay cells, including those of the lateral geniculate nucleus, display a low-threshold spike (LT spike), which is a large depolarization due to an increased Ca2+ conductance. Typically riding the crest of each LT spike is a burst of from two to seven action potentials, which we refer to as the LT burst. The LT spike is voltage dependent, because if the cell's resting membrane potential is more depolarized than roughly -60 mV, the LT spike is inactivated, but if more hyperpolarized, the spike is deinactivated and can be activated by a depolarization, such as from an afferent excitatory postsynaptic potential (EPSP). Thalamic relay cells thus display two response modes: a relay or tonic mode, when the cell is depolarized and LT spikes are inactivated, leading to tonic firing of action potentials; and a burst mode, when the cell is hyperpolarized and tends to respond with LT spikes and their associated bursts of action potentials. 2. We were interested in the contribution of the LT spike on the transmission of visually evoked signals through geniculate relay cells to visual cortex. We recorded intracellularly from geniculate cells in an anesthetized, paralyzed, in vivo cat preparation to study the effects of membrane voltage, and thus the presence or absence of LT spikes, on responses to drifting sine-wave gratings. We monitored the visually evoked responses of 14 geniculate neurons (6 X, 7 Y, and 1 unclassified) at different membrane potentials at which LT spikes were inactivated or deinactivated. 3. Changing membrane voltage during visual stimulation switched the response mode of every cell between the relay and burst modes. In the burst mode, LT spikes occurred in phase with the visual stimulus and not at rhythmic intervals uncorrelated to visual stimuli. To any given stimulus cycle, the cell responded usually with an LT burst or a tonic response, and rarely was more than one LT burst evoked by a stimulus cycle. Occasionally a single cycle evoked both an LT burst and tonic response, but always the LT burst occurred first. 4. The spatial tuning characteristics of the cells did not differ dramatically as a function of membrane potential, because the tuning of the LT bursts was quite similar to that of the tonic response component. Although we did not obtain complete temporal tuning properties, we did note that hyperpolarized cells responded reliably with LT bursts at several temporal frequencies. 5. A consistent difference was seen between the LT burst and tonic response components in terms of response linearity.(ABSTRACT TRUNCATED AT 400 WORDS)
Asunto(s)
Canales de Calcio/fisiología , Cuerpos Geniculados/fisiología , Neuronas Aferentes/fisiología , Potenciales de Acción/fisiología , Animales , Gatos , Electroencefalografía , Electrofisiología , Cuerpos Geniculados/citología , Potenciales de la Membrana/fisiología , Estimulación Luminosa , Percepción Espacial/fisiología , Tálamo/citología , Tálamo/fisiologíaRESUMEN
Neurons of the cat's dorsal lateral geniculate nucleus were recorded intracellularly to study the contribution of N-methyl-D-aspartate (NMDA) receptors to excitatory postsynaptic potentials (EPSPs) and low-threshold calcium spikes. EPSPs were evoked by stimulation of retinogeniculate axons in the optic tract and/or corticogeniculate axons in the optic radiations; EPSPs from both sources were similar. These EPSPs had one or two components, and the second component had several characteristics of NMDA receptor-mediated events. For example, EPSP amplitude decreased when neurons were hyperpolarized and increased when stimulus frequency was increased; these EPSPs could also be blocked reversibly by application of the selective NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV). We also studied the influence of NMDA receptors on low-threshold calcium spikes, which are large, voltage- and calcium-dependent depolarizations that are often accompanied by high-frequency action potential discharge. APV blocked synaptically activated low-threshold calcium spikes, but APV had no effect on low-threshold calcium spikes that were elicited by current injection. Therefore, APV does not appear to have a direct effect on the T-type calcium channel that is involved in generation of low-threshold calcium spikes. The voltage and frequency dependence of the NMDA receptor-mediated component of the EPSPs, as well as its ability to trigger low-threshold calcium spikes, provide for complex signal processing in the lateral geniculate nucleus.