Effects of bicuculline on signal detectability in lateral geniculate nucleus relay cells

Group I metabotropic glutamate receptors in the primate motor thalamus: subsynaptic association with cortical and sub-cortical glutamatergic afferents

Preclinical evidence indicates that mGluR5 is a potential therapeutic target for Parkinson's disease and L-DOPA-induced dyskinesia. However, the mechanisms through which these therapeutic benefits are mediated remain poorly understood. Although the regulatory role of mGluR5 on glutamatergic transmission has been examined in various basal ganglia nuclei, very little is known about the localization and function of mGluR5 in the ventral motor and intralaminar thalamic nuclei, the main targets of basal ganglia output in mammals. Thus, we used immuno-electron microscopy to map the cellular and subcellular localization of group I mGluRs (mGluR1a and mGluR5) in the ventral motor and caudal intralaminar thalamic nuclei in rhesus monkeys. Furthermore, using double immuno-electron microscopy, we examined the subsynaptic localization of mGluR5 in relation to cortical and sub-cortical glutamatergic afferents. Four major conclusions can be drawn from these data. First, mGluR1a and mGluR5 are expressed postsynaptically on the plasma membrane of dendrites of projection neurons and GABAergic interneurons in the basal ganglia- and cerebellar-receiving regions of the ventral motor thalamus and in CM. Second, the plasma membrane-bound mGluR5 immunoreactivity is preferentially expressed perisynaptically at the edges of cortical and sub-cortical glutamatergic afferents. Third, the mGluR5 immunoreactivity is more strongly expressed in the lateral than the medial tiers of CM, suggesting a preferential association with thalamocortical over thalamostriatal neurons in the primate CM. Overall, mGluR5 is located to subserve powerful modulatory role of cortical and subcortical glutamatergic transmission in the primate ventral motor thalamus and CM.

Short-term Synaptic Depression in the Feedforward Inhibitory Circuit in the Dorsal Lateral Geniculate Nucleus

Synaptic short-term plasticity (STP) regulates synaptic transmission in an activity-dependent manner and thereby has important roles in the signal processing in the brain. In some synapses, a presynaptic train of action potentials elicits post-synaptic potentials that gradually increase during the train (facilitation), but in other synapses, these potentials gradually decrease (depression). We studied STP in neurons in the visual thalamic relay, the dorsal lateral geniculate nucleus (dLGN). The dLGN contains two types of neurons: excitatory thalamocortical (TC) neurons, which transfer signals from retinal afferents to visual cortex, and local inhibitory interneurons, which form an inhibitory feedforward loop that regulates the thalamocortical signal transmission. The overall STP in the retino-thalamic relay is short-term depression, but the distinct kind and characteristics of the plasticity at the different types of synapses are unknown. We studied STP in the excitatory responses of interneurons to stimulation of retinal afferents, in the inhibitory responses of TC neurons to stimulation of afferents from interneurons, and in the disynaptic inhibitory responses of TC neurons to stimulation of retinal afferents. Moreover, we studied STP at the direct excitatory input to TC neurons from retinal afferents. The STP at all types of the synapses showed short-term depression. This depression can accentuate rapid changes in the stream of signals and thereby promote detectability of significant features in the sensory input. In vision, detection of edges and contours is essential for object perception, and the synaptic short-term depression in the early visual pathway provides important contributions to this detection process.

Distinct kinetics of inhibitory currents in thalamocortical neurons that arise from dendritic or axonal origin

Thalamocortical neurons in the dorsal lateral geniculate nucleus (dLGN) transfer visual information from retina to primary visual cortex. This information is modulated by inhibitory input arising from local interneurons and thalamic reticular nucleus (TRN) neurons, leading to alterations of receptive field properties of thalamocortical neurons. Local GABAergic interneurons provide two distinct synaptic outputs: axonal (F1 terminals) and dendritic (F2 terminals) onto dLGN thalamocortical neurons. By contrast, TRN neurons provide only axonal output (F1 terminals) onto dLGN thalamocortical neurons. It is unclear if GABA_A receptor-mediated currents originating from F1 and F2 terminals have different characteristics. In the present study, we examined multiple characteristics (rise time, slope, halfwidth and decay τ) of GABA_A receptor-mediated miniature inhibitory postsynaptic synaptic currents (mIPSCs) originating from F1 and F2 terminals. The mIPSCs arising from F2 terminals showed slower kinetics relative to those from F1 terminals. Such differential kinetics of GABA_AR-mediated responses could be an important role in temporal coding of visual signals.

The multifaceted role of inhibitory interneurons in the dorsal lateral geniculate nucleus

Intrinsic interneurons within the dorsal lateral geniculate nucleus (dLGN) provide a feed-forward inhibitory pathway for afferent visual information originating from the retina. These interneurons are unique because in addition to traditional axodendritic output onto thalamocortical neurons, these interneurons have presynaptic dendrites that form dendrodendritic synapses onto thalamocortical neurons as well. These presynaptic dendrites, termed F2 terminals, are tightly coupled to the retinogeniculate afferents that synapse onto thalamocortical relay neurons. Retinogeniculate stimulation of F2 terminals can occur through the activation of ionotropic and/or metabotropic glutamate receptors. The stimulation of ionotropic glutamate receptors can occur with single stimuli and produces a short-lasting inhibition of the thalamocortical neuron. By contrast, activation of metabotropic glutamate receptors requires tetanic activation and results in longer-lasting inhibition in the thalamocortical neuron. The F2 terminals are predominantly localized to the distal dendrites of interneurons, and the excitation and output of F2 terminals can occur independent of somatic activity within the interneuron thereby allowing these F2 terminals to serve as independent processors, giving rise to focal inhibition. By contrast, strong transient depolarizations at the soma can initiate a backpropagating calcium-mediated potential that invades the dendritic arbor activating F2 terminals and leading to a global form of inhibition. These distinct types of output, focal versus global, could play an important role in the temporal and spatial roles of inhibition that in turn impacts thalamocortical information processing.

Complex regulation of dendritic transmitter release from thalamic interneurons

Neuronal output typically involves neurotransmitter release via axonal terminals; however, a subpopulation of neurons can also release neurotransmitters through vesicle-containing presynaptic dendrites. In the thalamus, local circuit inhibitory interneurons are a class of cells that can release γ-aminobutyric acid (GABA) via both axon terminals (termed F1 terminals) as well as presynaptic, vesicle-containing dendrites (termed F2 terminals). For example, in the visual thalamus, these F2 terminals are tightly coupled to the primary sensory afferents (axons of retinogeniculate neurons) that synapse onto thalamocortical relay neurons. The F2 terminals are primarily localized to distal dendrites of the interneurons, and in certain situations the excitation/output of F2 terminals can occur independent of somatic activity within the interneuron thereby allowing these F2 terminals to serve as independent input/output components giving rise to focal inhibition. On the other hand, somatically evoked Na+-dependent action potentials can backpropagate throughout the dendritic arbor of the interneuron. The transient depolarizations, or stronger somatically initiated events (e.g. activation of low threshold calcium transients) can initiate a backpropagating Ca(2+)-mediated potential that invades the dendritic arbor activating F2 terminals and leading to a global form of inhibition. These distinct types of output (focal versus global) could play an important role in the temporal as well as spatial roles of inhibition that in turn impacts thalamocortical information processing.

Protein kinase C regulates tonic GABA(A) receptor-mediated inhibition in the hippocampus and thalamus

Tonic inhibition mediated by extrasynaptic GABA_A receptors (GABA_ARs) is an important regulator of neuronal excitability. Phosphorylation by protein kinase C (PKC) provides a key mode of regulation for synaptic GABA_ARs underlying phasic inhibition; however, less attention has been focused on the plasticity of tonic inhibition and whether this can also be modulated by receptor phosphorylation. To address this issue, we used whole-cell patch clamp recording in acute murine brain slices at both room and physiological temperatures to examine the effects of PKC-mediated phosphorylation on tonic inhibition. Recordings from dentate gyrus granule cells in the hippocampus and dorsal lateral geniculate relay neurons in the thalamus demonstrated that PKC activation caused downregulation of tonic GABA_AR-mediated inhibition. Conversely, inhibition of PKC resulted in an increase in tonic GABA_AR activity. These findings were corroborated by experiments on human embryonic kidney 293 cells expressing recombinant α4β2δ GABA_ARs, which represent a key extrasynaptic GABA_AR isoform in the hippocampus and thalamus. Using bath application of low GABA concentrations to mimic activation by ambient neurotransmitter, we demonstrated a similar inhibition of receptor function following PKC activation at physiological temperature. Live cell imaging revealed that this was correlated with a loss of cell surface GABA_ARs. The inhibitory effects of PKC activation on α4β2δ GABA_AR activity appeared to be mediated by direct phosphorylation at a previously identified site on the β2 subunit, serine 410. These results indicate that PKC-mediated phosphorylation can be an important physiological regulator of tonic GABA_AR-mediated inhibition.

Local dendrodendritic inhibition regulates fast synaptic transmission in visual thalamus

Inhibition from thalamic interneurons plays a critical role in modulating information transfer between thalamus and neocortex. Interestingly, these neurons yield inhibition via two distinct outputs: presynaptic dendrites that innervate thalamocortical relay neurons and axonal outputs. Since the dendrites of thalamic interneurons are the primary targets of incoming synaptic information, it has been hypothesized that local synaptic input could produce highly focused dendritic output. To gain additional insight into the computational power of these presynaptic dendrites, we have combined two-photon laser scanning microscopy, glutamate uncaging, and whole-cell electrophysiological recordings to locally activate dendritic terminals and study their inhibitory contribution to rat thalamocortical relay neurons. Our findings demonstrate that local dendritic release from thalamic interneurons is controlled locally by AMPA/NMDA receptor-mediated recruitment of L-type calcium channels. Moreover, by mapping these connections with single dendrite resolution we not only found that presynaptic dendrites preferentially target proximal regions, but such actions differ significantly across branches. Furthermore, local stimulation of interneuron dendrites did not result in global excitation, supporting the notion that these interneurons can operate as multiplexors, containing numerous independently operating input-output devices.

mGluR control of interneuron output regulates feedforward tonic GABAA inhibition in the visual thalamus

Metabotropic glutamate receptors (mGluRs) play a crucial role in regulation of phasic inhibition within the visual thalamus. Here we demonstrate that mGluR-dependent modulation of interneuron GABA release results in dynamic changes in extrasynaptic GABA(A) receptor (eGABA(A)R)-dependent tonic inhibition in thalamocortical (TC) neurons of the rat dorsal lateral geniculate nucleus (dLGN). Application of the group I selective mGluR agonist dihydroxyphenylglycine produces a concentration-dependent enhancement of both IPSC frequency and tonic GABA(A) current (I(GABA)tonic) that is due to activation of both mGluR1a and mGluR5 subtypes. In contrast, group II/III mGluR activation decreases both IPSC frequency and I(GABA)tonic amplitude. Using knock-out mice, we show that the mGluR-dependent modulation of I(GABA)tonic is dependent upon expression of δ-subunit containing eGABA(A)Rs. Furthermore, unlike the dLGN, no mGluR-dependent modulation of I(GABA)tonic is present in TC neurons of the somatosensory ventrobasal thalamus, which lacks GABAergic interneurons. In the dLGN, enhancement of IPSC frequency and I(GABA)tonic by group I mGluRs is not action potential dependent, being insensitive to TTX, but is abolished by the L-type Ca(2+) channel blocker nimodipine. These results indicate selective mGluR-dependent modulation of dendrodendritic GABA release from F2-type terminals on interneuron dendrites and demonstrate for the first time the presence of eGABA(A)Rs on TC neuron dendritic elements that participate in "triadic" circuitry within the dLGN. These findings present a plausible novel mechanism for visual contrast gain at the thalamic level and shed new light upon the potential role of glial ensheathment of synaptic triads within the dLGN.

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.

Contrary roles of kainate receptors in transmitter release at corticothalamic synapses onto thalamic relay and reticular neurons

Corticothalamic fibres, which originate from layer VI pyramidal neurons in the cerebral cortex, provide excitatory synaptic inputs to both thalamic relay neurons and reticular neurons; reticular neurons in turn supply inhibitory inputs to thalamic relay neurons. Pyramidal cells in layer VI in the mouse somatosensory cortex highly express mRNA encoding kainate receptors, which facilitate or depress transmitter release at several synapses in the central nervous system. We report here that contrary modulation of transmitter release from corticothalamic fibres onto thalamic relay and reticular neurons is mediated by activation of kainate receptors in mouse thalamic ventrobasal complex and thalamic reticular nucleus. Exogenous kainate presynaptically depresses the synaptic transmission at corticothalamic synapses onto thalamic relay neurons, but facilitates it at corticothalamic synapses onto reticular neurons. Meanwhile, the lemniscal synaptic transmission, which sends primary somatosensory inputs to relay neurons, is not affected by kainate. In addition, GluR5-containing kainate receptors are involved in the depression of corticothalamic synaptic transmission onto relay neurons, but not onto reticular neurons. Furthermore, synaptically activated kainate receptors mimic these effects; high-frequency stimulation of corticothalamic fibres depresses synaptic transmission onto relay neurons, but facilitates it onto reticular neurons. Our results suggest that the opposite sensitivity of kainate receptors at the two corticothalamic synapses is governed by cortical activity and regulates the balance of excitatory and inhibitory inputs to thalamic relay neurons and therefore their excitability.

Excitatory and anti-oscillatory actions of nitric oxide in thalamus

The dorsal lateral geniculate nucleus (dLGN) not only serves as the obligatory pathway for visual information transfer from the retina to neocortex but can also generate intrathalamic rhythmic activities associated with different arousal states and certain pathological conditions. The gating activity of thalamocortical circuits is under neuromodulatory control by various brainstem nuclei as well as intrinsic thalamic neurons (e.g. thalamic reticular nucleus (TRN) neurons and dLGN interneurons). In this study, we examined the effect of the putative neuromodulator nitric oxide (NO) on thalamic neuron excitability. There are multiple potential sources of NO in thalamus: cholinergic terminals originating from brainstem nuclei, GABAergic TRN neurons, and local GABAergic interneurons. Using whole cell recording techniques in in vitro thalamic slices, we found that the NO donor SNAP produced a robust, long-lasting depolarization in TRN neurons, a weaker depolarization in thalamocortical relay neurons, and no effect in local interneurons. SNAP preferentially depolarized stereotypical TRN neurons that could produced strong burst discharge. In contrast, SNAP had little effect on atypical burst and non-burst TRN cells. The NO donor SIN-1 and the endogenous NO precursor, L-arginine, mimicked the SNAP-mediated actions. The NO-mediated depolarizations were blocked by the guanylyl cyclase inhibitor ODQ indicating involvement of the cGMP pathway. In addition, the phosphodiesterase (PDE) inhibitor zaprinast depolarized and occluded the NO-mediated depolarization in TRN neurons. At the circuit level, NO activation significantly attenuated intrathalamic rhythmic activities likely resulting from the shifting of the firing mode of thalamic neurons, perhaps both TRN and thalamocortical neurons, from burst- to tonic-discharge mode. These alterations in thalamic neuron excitability not only change rhythmic circuit activity, but could also influence sensory information processing through thalamocortical circuits.

Modulation of inhibitory activity by nitric oxide in the thalamus

The dorsal lateral geniculate nucleus (dLGN) is essential for the transfer of visual information from the retina to visual cortex, and inhibitory mechanisms can play a critical in regulating such information transfer. Nitric oxide (NO) is an atypical neuromodulator that is released in gaseous form and can alter neural activity without direct synaptic connections. Nitric oxide synthase (NOS), an essential enzyme for NO production, is localized in thalamic inhibitory neurons and cholinergic brain stem neurons that innervate the thalamus, although NO-mediated effects on thalamic inhibitory activity remain unknown. We investigated NO effects on inhibitory activity in dLGN using an in vitro slice preparation. The NO donor, SNAP, selectively potentiated the frequency, but not amplitude, of spontaneous inhibitory postsynaptic currents (sIPSCs) in thalamocortical relay neurons. This increase also persisted in tetrodotoxin (TTX), consistent with an increase in GABA release from presynaptic terminals. The SNAP-mediated actions were attenuated not only by the NO scavenger carboxy-PTIO but also by the guanylyl cyclase inhibitor ODQ. The endogenous NO precursor L-arginine produced actions similar to those of SNAP on sIPSC activity and these L-arginine-mediated actions were attenuated by the NOS inhibitor L-NMMA acetate. The SNAP-mediated increase in sIPSC activity was observed in both dLGN and ventrobasal thalamic nucleus (VB) neurons. Considering the lack of interneurons in rodent VB, the NO-mediated actions likely involve an increase in the output of axon terminals of thalamic reticular nucleus neurons. Our results indicate that NO upregulates thalamic inhibitory activity and thus these actions likely influence sensory information transfer through thalamocortical circuits.

Metabotropic glutamate receptors differentially regulate GABAergic inhibition in thalamus

Thalamic interneurons and thalamic reticular nucleus (TRN) neurons provide inhibitory innervation of thalamocortical cells that significantly influence thalamic gating. The local interneurons in the dorsal lateral geniculate nucleus (dLGN) give rise to two distinct synaptic outputs: classical axonal and dendrodendritic. Activation of metabotropic glutamate receptors (mGluRs) by agonists or optic tract stimulation increases the output of these presynaptic dendrites leading to increased inhibition of thalamocortical neurons. The present study was aimed to evaluate the actions of specific mGluRs on inhibitory GABA-mediated signaling. We found that the group I mGluR (mGluR(1,5)) agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) or optic tract stimulation produced a robust increase in spontaneous IPSCs (sIPSCs) in thalamocortical neurons that was attenuated by the selective mGluR(5) antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP). In contrast, the group II mGluR (mGluR(2,3)) agonists (2R, 4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) or (2S,2'R,3'R)-2-(2'3'-dicarboxycyclopropyl)glycine (DCG-IV) suppressed the frequency of sIPSCs. In addition, mGluR(1,5) agonist DHPG produced depolarizations and mGluR(2/3) agonists APDC or L-CCG-I [(2S,1'S,2'S)-2-(carboxycyclopropyl)glycine] produced hyperpolarizations in dLGN interneurons. Furthermore, the enhanced sIPSC activity by optic tract stimulation was reduced when paired with corticothalamic fiber stimulation. The present data indicate that activation of specific mGluR subtypes differentially regulates inhibitory activity via different synaptic pathways. Our results suggest that activation of specific mGluR subtypes can upregulate or downregulate inhibitory activity in thalamic relay neurons, and these actions likely shape excitatory synaptic integration and thus regulate information transfer through thalamocortical circuits.

Chronic morphine exposure induces degradation of receptive field properties of LGN cells in cats

AIM

To investigate the effect of chronic morphine exposure on the receptive field properties of lateral geniculate nucleus (LGN) neurons in cats.

METHODS

Cats were injected with morphine (10 mg/kg) or saline twice daily, for 10 d. Subsequently, extracellular single-unit recording techniques were used to examine the sensitivity of LGN neurons to visual stimuli in chronic morphine-treated and saline-treated cats.

RESULTS

Compared with saline-treated cats (as controls), LGN neurons in morphine-treated cats had decreased signal-to-noise ratios (1.9 vs 3.1), and orientation and direction sensitivity (0.103 vs 0.135 and 0.074 vs 0.10, respectively), accompanied by significant increases in spontaneous (27.4 vs 17.5) and evoked activity (preferred: 42.2 vs 38.1; average: 28.1 vs 22.3).

CONCLUSION

Chronic morphine exposure can lead to the functional degradation of LGN neurons in cats, which might result from the effects of chronic morphine exposure on inhibitory neurotransmission.

Timing and specificity of feed-forward inhibition within the LGN

Local interneurons provide feed-forward inhibition from retinal ganglion cells (RGCs) to thalamocortical (TC) neurons, but questions remain regarding the timing, magnitude, and functions of this inhibition. Here, we identify two types of inhibition that are suited to play distinctive roles. We recorded excitatory and inhibitory postsynaptic currents (EPSCs/IPSCs) in TC neurons in mouse brain slices and activated individual RGC inputs. In 34% of TC neurons, we identified EPSCs and IPSCs with identical thresholds that were tightly correlated, indicating activation by the same RGC. Such "locked" IPSCs occurred 1 ms after EPSC onset. The remaining neurons had only "nonlocked" inhibition, in which EPSCs and IPSCs had different thresholds, indicating activation by different RGCs. Nonlocked inhibition may refine receptive fields within the LGN by providing surround inhibition. In contrast, dynamic-clamp recordings suggest that locked inhibition improves the precision of synaptically evoked responses in individual TC neurons by eliminating secondary spikes.

Movement-related discharge in the cerebellar nuclei persists after local injections of GABA(A) antagonists

Limb movement-related neurons in the cerebellar nuclei (CN) typically produce bursts of discharge in association with movement. Consequently, given the inhibitory nature of the Purkinje cell (PC) projection to CN, it is puzzling that only a minority of movement-related PCs pause; the majority burst. Some of the movement-related CN activity may be the result of excitation from collaterals of mossy and climbing fiber projections to the cerebellar cortex. The only other input to CN is diffuse and neuromodulatory, from locus ceruleus and raphe nuclei. To investigate the role of the excitatory mossy fiber input, single units in CN were recorded in macaque monkeys during the performance of reaching and manipulation tasks, before and after blocking the PC input with local microinjections of GABA(A) antagonists (bicuculline or SR95531). After these injections, the movement-related modulation of CN discharge was greater and began earlier, compared with the modulation in the preinjection group of neurons. These observations indicate that an important excitatory drive is provided by extracerebellar inputs to CN, most likely from collaterals of mossy fibers. PCs may serve primarily to regulate this activity, by either pausing or bursting as necessary.

Synaptic Activation of Metabotropic Glutamate Receptors Regulates Dendritic Outputs of Thalamic Interneurons

Information gating through the thalamus is dependent on the output of thalamic relay neurons. These relay neurons receive convergent innervation from a number of sources, including GABA-containing interneurons that provide feed-forward inhibition. These interneurons are unique in that they have two distinct outputs: axonal and dendritic. In addition to conventional axonal outputs, these interneurons have presynaptic dendrites that may provide localized inhibitory influences. Our study indicates that synaptic activation of metabotropic glutamate receptors (mGluRs) increases inhibitory activity in relay neurons by increasing output of presynaptic dendrites of interneurons. Optic tract stimulation increases inhibitory activity in thalamic relay neurons in a frequency- and intensity-dependent manner and is attenuated by mGluR antagonists. Our data suggest that synaptic activation of mGluRs selectively alters dendritic output but not axonal output of thalamic interneurons. This mechanism could serve an important role in focal, feed-forward information processing in addition to dynamic information processing in thalamocortical circuits.

Effects of bicuculline on direction-sensitive relay cells in the dorsal lateral geniculate nucleus (LGNd) of cats

The direction sensitivity of relay cells in the cat's dorsal lateral geniculate (LGNd) was measured using sinusoidal grating stimuli before and during local bicuculline administration. One hundred and twenty-eight LGNd relay cells were recorded in laminae A and A1, of which 44 relay cells (34%) were found to be sensitive to direction of stimulus movement. The direction-sensitive LGNd relay cells could be differentiated into two subgroups based on different measures of their response amplitude. Type I cells exhibited their direction sensitivity when the fundamental Fourier component (FFC) of the poststimulus time histograms (PSTHs) was used as response measure, but did not show significant direction sensitivity when mean firing rate was used. Type II cells exhibited their direction sensitivity, no matter whether the FFC or mean firing rate was used as the measure. Of 35 cells analyzed, 27 cells remained direction sensitive during bicuculline administration. At the population level, the direction bias of type I cells did not change systematically, while the direction bias of type II cells decreased significantly during bicuculline administration. These results suggest that the direction bias of these two types of relay cells are mediated by different neural mechanisms. The direction bias of type I cells may involve multiple inputs from spatio-temporally separate subunits within retinal ganglion cells receptive fields. The direction bias of type II cells may involve GABAergic neuronal circuits within the LGNd.

Quantitative analyses of synaptic contacts of interneurons in the dorsal lateral geniculate nucleus of the squirrel monkey

Three interneurons were recorded from and then injected with horseradish peroxidase in the parvocellular laminae of the squirrel monkey's (Saimiri sciureus) dorsal lateral geniculate nucleus. They were then examined using the electron microscope for their synaptic contacts, both the afferent contacts onto their dendrites and their presynaptic dendritic contacts onto presumptive projection (relay) neuron dendrites. The somata of these interneurons were small (mean = 178 microns 2), but the dendritic trees were large compared with those of projection neurons. All three interneurons had similar synaptic patterns onto their dendrites with about equal numbers of retinal, cortical, and GABAergic contacts. The distribution of these contacts was more uniform compared with the same types of contacts made onto projection neurons. The presynaptic dendrites were observed to contact only the dendrites of presumptive projection neurons, and these contacts were nearly all in the form of geniculate triads. None of the three interneurons displayed an axon. The receptive fields of these interneurons were similar to those of projection cells, but were larger and had center-response signs that were the opposite of the projection neurons around them (e.g. OFF center for the dorsal part of the parvocellular mass where ON-center projection neurons reside). The squirrel monkey data provides additional evidence that one aspect of the laminar pattern observed in the parvocellular pathway of the primate's dLGN might be related to a segregation of projection neurons of one center-response sign with interneurons of the opposite center-response sign.

Electrophysiological and pharmacological properties of interneurons in the cat dorsal lateral geniculate nucleus

We investigated the electrophysiological and pharmacological properties of morphologically identified and putative interneurons within laminae A and A1 of the cat dorsal lateral geniculate nucleus maintained in vitro. These intralaminar interneurons possess unique electrophysiological characteristics, including (1) action potentials of a short duration (average width at half amplitude of 0.34 ms). (2) the ability to generate high-frequency trains of action potentials exceeding 500 Hz, without strong spike frequency adaptation, and (3) a low-threshold regenerative response with variable magnitude of expression, ranging from a subthreshold depolarization towards the generation of one to several action potentials in different cells. The low-threshold regenerative depolarization following a hyperpolarizing current pulse was increased in size by application of 4-aminopyridine, was reduced by nickel, and was not influenced by extracellular cesium. These findings indicate that this event is mediated by an underlying Ca(2+)-dependent mechanism, such as a low-threshold Ca(2+) current, that is regulated by the activation of opposing transient K+ currents. Every interneuron tested responded to glutamate, kainate, quisqualate, or N-methyl-D-aspartate with depolarization and action potential discharge. In contrast, we did not observe a postsynaptic response to activation of the metabotropic receptors with 1S,3R-(+/-)-1-amino-cyclopentane-1,3-dicarboxylate. Application of gamma-amino-butyric acid (GABA) strongly inhibited spike firing through a biphasic hyperpolarization and increase in membrane conductance, a response that reversed close to the presumed chloride equilibrium potential and was imitated by the GABAA receptor agonist muscimol. The GABAB receptor agonist baclofen evoked only a weak membrane hyperpolarization from rest and suppression of spontaneous spike activity. Application of acetylcholine, or the muscarinic agonist acetyl-beta-methylcholine, inhibited spontaneous action potential activity through hyperpolarization of the membrane potential, presumably resulting from an increase in membrane potassium conductance. In contrast, application of serotonin only slightly facilitated tonic activity in a subpopulation of interneurons, histamine induced a small slow depolarization apparently through activation of presynaptic excitatory pathways, and noradrenaline and adenosine had no detectable effect on the spontaneous firing or resting potential of interneurons. We suggest that intralaminar interneurons may function in a relatively linear manner to transform retinal and cortical inputs into a local field of inhibition in the dorsal lateral geniculate and that the excitability of these neurons is largely controlled by retinal, cortical, GABAergic, and cholinergic (brainstem) afferents.

Receiver operating characteristic (ROC) analysis of neurons in the cat's lateral geniculate nucleus during tonic and burst response mode

Relay cells of the lateral geniculate nucleus respond to visual stimuli in one of two modes: burst and tonic. The burst mode depends on the activation of a voltage-dependent, Ca2+ conductance underlying the low threshold spike. This conductance is inactivated at depolarized membrane potentials, but when activated from hyperpolarized levels, it leads to a large, triangular, nearly all-or-none depolarization. Typically, riding its crest is a high-frequency barrage of action potentials. Low threshold spikes thus provide a nonlinear amplification allowing hyperpolarized relay neurons to respond to depolarizing inputs, including retinal EPSPs. In contrast, the tonic mode is characterized by a steady stream of unitary action potentials that more linearly reflects the visual stimulus. In this study, we tested possible differences in detection between response modes of 103 geniculate neurons by constructing receiver operating characteristic (ROC) curves for responses to visual stimuli (drifting sine-wave gratings and flashing spots). Detectability was determined from the ROC curves by computing the area under each curve, known as the ROC area. Most cells switched between modes during recording, evidently due to small shifts in membrane potential that affected the activation state of the low threshold spike. We found that the more often a cell responded in burst mode, the larger its ROC area. This was true for responses to optimal and nonoptimal visual stimuli, the latter including nonoptimal spatial frequencies and low stimulus contrasts. The larger ROC areas associated with burst mode were due to a reduced spontaneous activity and roughly equivalent level of visually evoked response when compared to tonic mode. We performed a within-cell analysis on a subset of 22 cells that switched modes during recording. Every cell, whether tested with a low contrast or high contrast visual stimulus exhibited a larger ROC area during its burst response mode than during its tonic mode. We conclude that burst responses better support signal detection than do tonic responses. Thus, burst responses, while less linear and perhaps less useful in providing a detailed analysis of visual stimuli, improve target detection. The tonic mode, with its more linear response, seems better suited for signal analysis rather than signal detection.

Phasic stimulation of the locus coeruleus: effects on activity in the lateral geniculate nucleus

Neurons in the locus coeruleus (LC) encode information related to behavioral state in a tonic pattern of firing and information related to the occurrence of a sensory stimulus in a phasic pattern of firing. The effects of phasic stimulation of the LC (6 pulses at 30 Hz), designed to approximate its physiological activation by sensory stimuli, were studied in the lateral geniculate nucleus (LGN) of anesthetized rats. Phasic stimulation of the LC significantly increased neuronal firing in the LGN with a mean latency 320 ms from onset of stimulation. Receiver operating characteristic analyses on a trial-by-trial basis showed that phasic LC stimulation can result in a highly discriminable signal in the LGN. This increased neuronal firing rate in the LGN was specific for the site of stimulation and was reduced by the norepinephrine synthesis inhibitor alpha-methyl-p-tyrosine and by intravenous WB-4101 (alpha 1-receptor antagonist). Neurons in the LGN have a single-spike firing mode when sensory information is faithfully relayed from retina to cortex and a burst-firing mode when the transfer of this information is degraded. Phasic LC stimulation reduced burst firing (2-5 ms interspike intervals, ISIs) at low frequencies (< or = 4 Hz) in the LGN, and for some neurons there was an absolute decrease in burst-like ISIs after LC stimulation, despite an increase in mean firing rate.

Amino acids modify thalamo-cortical response transformation expressed by neurons of the ventrobasal complex

The hypothesis has been tested that inhibitory mechanisms, active spatially and temporally between the input and the output of thalamic neurons, determine the nature of the information transmitted to the cerebral cortex. To enable this assessment, in barbiturate-anesthetized cats and urethane-anesthetized rats juxtacellular recordings were performed together with microiontophoretic ejection of transmitter agonists and antagonists. The effects of these drugs were studied on responses evoked by mechanical stimulation of cutaneous receptive fields (RFs) of neurons in the thalamic ventrobasal complex (VB). Neurons from different parts of the VB were investigated: 29 units were located medially, in the ventral posteromedial nucleus (VPM; facial RFs), and 11 units were located laterally, in the ventral posterolateral nucleus (VPL; forepaw and body RFs). A further eleven VB units had no detectable RF. Twenty-six neurons were tested with electrical stimulation of the somatosensory cortex (SI), 17 of these being identified as thalamo-cortical relay neurons and 5 being classified as presumed interneurons; the remaining 4 could not be activated. Four additional recordings were from trigemino-thalamic or thalamo-cortical fibers. For the quantitative assessment of the neurons' input and output, neuronal activity was induced by feedback-controlled, mechanical trapezoidal and/or sinusoidal stimuli applied to sinus hairs, fur or skin and the numbers of prepotentials and soma spikes were compared in peristimulus time histograms (PSTHs) generated simultaneously for both types of signal from 'DC' recordings. Iontophoretic administration of excitatory amino acids (EAAs) or bicuculline methiodide (BMI) increased output-input ratios in 87% of the cases tested, due to a higher rate of conversion of prepotentials into soma spikes taking place. In cases of neurons exhibiting a sustained-to-transient response pattern, changes to sustained-to-sustained patterns were demonstrated. Tests with gamma-aminobutyric acid (GABA) produced decreased output-input ratios in 90% of the neurons, due to a lower conversion rate of prepotentials into soma spikes taking place. In cases of neurons exhibiting high output-input ratios (sustained-to-sustained type), the responses changed to the sustained-to-transient pattern. For cortically evoked antidromic spikes of VB neurons, GABA produced a failure of the initial segment (IS-) spike to invade the soma, whereas BMI and glutamate (Glu) facilitated soma depolarization. When ejected with relatively higher currents than those needed to alter output-input ratios, EAAs decreased prepotential amplitudes while GABA produced increases in 16 of 18 neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

A tungsten-in-glass iontophoresis assembly for studying input-output relationships in central neurons

A method is described for the production of an electrode capable of monitoring and modulating the input-output relationship of thalamic neurons. Tungsten-in-glass electrodes were manufactured with the ability to simultaneously record lateral geniculate nucleus action potentials and associated, retinally evoked S-potentials. The recording electrodes were mounted onto multibarreled micropipettes with iontophoretic capability. The completed electrode assembly permitted micropharmacological modulation of the fraction of lateral geniculate nucleus output spikes to retinal input spikes (the transfer ratio). Iontophoretically applied gamma-aminobutyric acid (GABA) decreased the transfer ratio, an effect countered by the GABAA antagonist, bicuculline. Elevated transfer ratios produced by stimulation of an afferent pathway originating in the parabrachial region of the brainstem were decreased by concurrently applied GABA. The fabrication of this electrode assembly employs simple modifications of existing techniques and separate construction of recording and iontophoretic elements to provide high-quality single-unit recordings coupled with micropharmacological function.

Effects of bicuculline on receptive field center sensitivity of relay cells in the lateral geniculate nucleus

The lateral geniculate nucleus (LGN) receives input from the retina that is spatially organized into a receptive-field center and surround. It maintains this organization in the signal that it sends to the visual cortex. Previous studies have focused on changes in the receptive-field 'surround' that are generated at the LGN, possibly as a local contrast enhancement mechanism. The present study suggests a role for the LGN in regulating the receptive-field center sensitivity under the control of GABAergic circuitry. Local microiontophoresis of the GABAA receptor antagonist bicuculline increased the contrast sensitivity of LGN relay cells to many spatial frequencies. Difference of Gaussians analysis showed that the increased was due to an increased sensitivity of the receptive-field center. Similar increases in receptive-field center sensitivity may be produced during behavioral arousal by the action of pontine and mesencephalic pathways upon the activity of the LGN GABAergic circuitry.