Mechanism of an Intrinsic Oscillation in Rat Geniculate Interneurons.

Depolarizing current injections produced a rhythmic bursting of action potentials - a bursting oscillation - in a set of local interneurons in the lateral geniculate nucleus (LGN) of rats. The current dynamics underlying this firing pattern have not been determined, though this cell type constitutes an important cellular component of thalamocortical circuitry, and contributes to both pathologic and non-pathologic brain states. We thus investigated the source of the bursting oscillation using pharmacological manipulations in LGN slices in vitro and in silico. 1. Selective blockade of calcium channel subtypes revealed that high-threshold calcium currents I L and I P contributed strongly to the oscillation. 2. Increased extracellular K+ concentration (decreased K+currents) eliminated the oscillation. 3. Selective blockade of K+ channel subtypes demonstrated that the calcium-sensitive potassium current ( I A H P ) was of primary importance. A morphologically simplified, multicompartment model of the thalamic interneuron characterized the oscillation as follows: 1. The low-threshold calcium current I T provided the strong initial burst characteristic of the oscillation. 2. Alternating fluxes through high-threshold calcium channels and I A H P then provided the continuing oscillation's burst and interburst periods respectively. This interplay between I L and I A H P contrasts with the current dynamics underlying oscillations in thalamocortical and reticularis neurons, which primarily involve I T and I H , or I T and I A H P respectively. These findings thus point to a novel electrophysiological mechanism for generating intrinsic oscillations in a major thalamic cell type. Because local interneurons can sculpt the behavior of thalamocortical circuits, these results suggest new targets for the manipulation of ascending thalamocortical network activity.

Evaluation of medial division of the medial geniculate (MGM) and posterior intralaminar nucleus (PIN) inputs to the rat auditory cortex, amygdala, and striatum.

The medial division of the medial geniculate (MGM) and the posterior intralaminar nucleus (PIN) are association nuclei of the auditory thalamus. We made tracer injections in these nuclei to evaluate/compare their presynaptic terminal and postsynaptic target features in auditory cortex, amygdala and striatum, at the light and electron microscopic levels. Cortical labeling was concentrated in Layer 1 but in other layers distribution was location-dependent. In cortical areas designated dorsal, primary and ventral (AuD, Au1, AuV) terminals deep to Layer 1 were concentrated in infragranular layers and sparser in the supragranular and middle layers. In ectorhinal cortex (Ect), distributions below Layer 1 changed with concentrations in supragranular and middle layers. In temporal association cortex (TeA) terminal distributions below Layer 1 was intermediate between AuV/1/D and Ect. In amygdala and striatum, terminal concentrations were higher in striatum but not as dense as in cortical Layer 1. Ultrastructurally, presynaptic terminal size was similar in amygdala, striatum or cortex and in all cortical layers. Postsynaptically MGM/PIN terminals everywhere synapsed on spines or small distal dendrites but as a population the postsynaptic structures in cortex were larger than those in the striatum. In addition, primary cortical targets of terminals were larger in primary cortex than in area Ect. Thus, although postsynaptic size may play some role in changes in synaptic influence between areas it appears that terminal size is not a variable used for that purpose. In auditory cortex, cortical subdivision-dependent changes in the terminal distribution between cortical layers may also play a role.

PV+ Cells Enhance Temporal Population Codes but not Stimulus-Related Timing in Auditory Cortex.

Spatio-temporal cortical activity patterns relative to both peripheral input and local network activity carry information about stimulus identity and context. GABAergic interneurons are reported to regulate spiking at millisecond precision in response to sensory stimulation and during gamma oscillations; their role in regulating spike timing during induced network bursts is unclear. We investigated this issue in murine auditory thalamo-cortical (TC) brain slices, in which TC afferents induced network bursts similar to previous reports in vivo. Spike timing relative to TC afferent stimulation during bursts was poor in pyramidal cells and SOM+ interneurons. It was more precise in PV+ interneurons, consistent with their reported contribution to spiking precision in pyramidal cells. Optogenetic suppression of PV+ cells unexpectedly improved afferent-locked spike timing in pyramidal cells. In contrast, our evidence suggests that PV+ cells do regulate the spatio-temporal spike pattern of pyramidal cells during network bursts, whose organization is suited to ensemble coding of stimulus information. Simulations showed that suppressing PV+ cells reduces the capacity of pyramidal cell networks to produce discriminable spike patterns. By dissociating temporal precision with respect to a stimulus versus internal cortical activity, we identified a novel role for GABAergic cells in regulating information processing in cortical networks.

Preferential effect of isoflurane on top-down vs. bottom-up pathways in sensory cortex.

The mechanism of loss of consciousness (LOC) under anesthesia is unknown. Because consciousness depends on activity in the cortico-thalamic network, anesthetic actions on this network are likely critical for LOC. Competing theories stress the importance of anesthetic actions on bottom-up "core" thalamo-cortical (TC) vs. top-down cortico-cortical (CC) and matrix TC connections. We tested these models using laminar recordings in rat auditory cortex in vivo and murine brain slices. We selectively activated bottom-up vs. top-down afferent pathways using sensory stimuli in vivo and electrical stimulation in brain slices, and compared effects of isoflurane on responses evoked via the two pathways. Auditory stimuli in vivo and core TC afferent stimulation in brain slices evoked short latency current sinks in middle layers, consistent with activation of core TC afferents. By contrast, visual stimuli in vivo and stimulation of CC and matrix TC afferents in brain slices evoked responses mainly in superficial and deep layers, consistent with projection patterns of top-down afferents that carry visual information to auditory cortex. Responses to auditory stimuli in vivo and core TC afferents in brain slices were significantly less affected by isoflurane compared to responses triggered by visual stimuli in vivo and CC/matrix TC afferents in slices. At a just-hypnotic dose in vivo, auditory responses were enhanced by isoflurane, whereas visual responses were dramatically reduced. At a comparable concentration in slices, isoflurane suppressed both core TC and CC/matrix TC responses, but the effect on the latter responses was far greater than on core TC responses, indicating that at least part of the differential effects observed in vivo were due to local actions of isoflurane in auditory cortex. These data support a model in which disruption of top-down connectivity contributes to anesthesia-induced LOC, and have implications for understanding the neural basis of consciousness.

Spiking in auditory cortex following thalamic stimulation is dominated by cortical network activity.

The state of the sensory cortical network can have a profound impact on neural responses and perception. In rodent auditory cortex, sensory responses are reported to occur in the context of network events, similar to brief UP states, that produce "packets" of spikes and are associated with synchronized synaptic input (Bathellier et al., 2012; Hromadka et al., 2013; Luczak et al., 2013). However, traditional models based on data from visual and somatosensory cortex predict that ascending sensory thalamocortical (TC) pathways sequentially activate cells in layers 4 (L4), L2/3, and L5. The relationship between these two spatio-temporal activity patterns is unclear. Here, we used calcium imaging and electrophysiological recordings in murine auditory TC brain slices to investigate the laminar response pattern to stimulation of TC afferents. We show that although monosynaptically driven spiking in response to TC afferents occurs, the vast majority of spikes fired following TC stimulation occurs during brief UP states and outside the context of the L4>L2/3>L5 activation sequence. Specifically, monosynaptic subthreshold TC responses with similar latencies were observed throughout layers 2-6, presumably via synapses onto dendritic processes located in L3 and L4. However, monosynaptic spiking was rare, and occurred primarily in L4 and L5 non-pyramidal cells. By contrast, during brief, TC-induced UP states, spiking was dense and occurred primarily in pyramidal cells. These network events always involved infragranular layers, whereas involvement of supragranular layers was variable. During UP states, spike latencies were comparable between infragranular and supragranular cells. These data are consistent with a model in which activation of auditory cortex, especially supragranular layers, depends on internally generated network events that represent a non-linear amplification process, are initiated by infragranular cells and tightly regulated by feed-forward inhibitory cells.

Thalamocortical projections to rat auditory cortex from the ventral and dorsal divisions of the medial geniculate nucleus.

The ventral and dorsal medial geniculate (MGV and MGD) constitute the major auditory thalamic subdivisions providing thalamocortical inputs to layer IV and lower layer III of auditory cortex. No quantitative evaluation of this projection is available. Using biotinylated dextran amine (BDA)/biocytin injections, we describe the cortical projection patterns of MGV and MGD cells. In primary auditory cortex the bulk of MGV axon terminals are in layer IV/lower layer III with minor projections to supragranular layers and intermediate levels in infragranular layers. MGD axons project to cortical regions designated posterodorsal (PD) and ventral (VA) showing laminar terminal distributions that are quantitatively similar to the MGV-to-primary cortex terminal distribution. At the electron microscopic level MGV and MGD terminals are non-γ-aminobutyric acid (GABA)ergic with MGD terminals in PD and VA slightly but significantly larger than MGV terminals in primary cortex. MGV/MGD terminals synapse primarily onto non-GABAergic spines/dendrites. A small number synapse on GABAergic structures, contacting large dendrites or cell bodies primarily in the major thalamocortical recipient layers. For MGV projections to primary cortex or MGD projections to PD or VA, the non-GABAergic postsynaptic structures at each site were the same size regardless of whether they were in supragranular, granular, or infragranular layers. However, the population of MGD terminal-recipient structures in VA were significantly larger than the MGD terminal-recipient structures in PD or the MGV terminal-recipient structures in primary cortex. Thus, if terminal and postsynaptic structure size indicate strength of excitation then MGD to VA inputs are strongest, MGD to PD intermediate, and MGV to primary cortex the weakest.

Descending projections from extrastriate visual cortex modulate responses of cells in primary auditory cortex.

Primary sensory cortical responses are modulated by the presence or expectation of related sensory information in other modalities, but the sources of multimodal information and the cellular locus of this integration are unclear. We investigated the modulation of neural responses in the murine primary auditory cortical area Au1 by extrastriate visual cortex (V2). Projections from V2 to Au1 terminated in a classical descending/modulatory pattern, with highest density in layers 1, 2, 5, and 6. In brain slices, whole-cell recordings revealed long latency responses to stimulation in V2L that could modulate responses to subsequent white matter (WM) stimuli at latencies of 5-20 ms. Calcium responses imaged in Au1 cell populations showed that preceding WM with V2L stimulation modulated WM responses, with both summation and suppression observed. Modulation of WM responses was most evident for near-threshold WM stimuli. These data indicate that corticocortical projections from V2 contribute to multimodal integration in primary auditory cortex.

Evaluation of inputs to rat primary auditory cortex from the suprageniculate nucleus and extrastriate visual cortex.

Evidence indicates that visual stimuli influence cells in the primary auditory cortex. To evaluate potential sources of this visual input and how they enter into the circuitry of the auditory cortex, we examined axonal terminations in the primary auditory cortex from nonprimary extrastriate visual cortex (V2M, V2L) and from the multimodal thalamic suprageniculate nucleus (SG). Gross biocytin/biotinylated dextran amine (BDA) injections into the SG or extrastriate cortex labeled inputs terminating primarily in superficial and deep layers. SG projects primarily to layers I, V, and VI while V2M and V2L project primarily to layers I and VI, with V2L also targeting layers II/III. Layer I inputs differ in that SG terminals are concentrated superficially, V2L are deeper, and V2M are equally distributed throughout. Individual axonal reconstructions document that single axons can 1) innervate multiple layers; 2) run considerable distances in layer I; and 3) run preferentially in the dorsoventral direction similar to isofrequency axes. At the electron microscopic level, SG and V2M terminals 1) are the same size regardless of layer; 2) are non-gamma-aminobutyric acid (GABA)ergic; 3) are smaller than ventral medial geniculate terminals synapsing in layer IV; 4) make asymmetric synapses onto dendrites/spines that 5) are non-GABAergic and 6) are slightly larger in layer I. Thus, both areas provide a substantial feedback-like input with differences that may indicate potentially different roles.

Computer model of passive signal integration based on whole-cell in vitro studies of rat lateral geniculate nucleus.

Computer models were used to investigate passive properties of lateral geniculate nucleus thalamocortical cells and thalamic interneurons based on in vitro whole-cell study. Two neurons of each type were characterized physiologically and morphologically. Thalamocortical cells transmitted 37% of steady-state signal orthodromically (distal dendrite to soma) and 93% antidromically (soma to distal dendrite); interneurons transmitted 18% orthodromically and 53% antidromically. Lowering membrane resistance caused a dramatic drop in steady-state signal transmission. Simulation of brief signals such as orthodromically transmitted postsynaptic potentials and antidromically transmitted action potentials showed relatively poor transmission due to the low-pass filtering property of dendrites. This attenuation was particularly pronounced in interneurons. By contrast, bursts of postsynaptic potentials or action potentials were relatively well transmitted as the temporal summation of these recurring signals gave prolonged depolarizations comparable to prolonged current injection. While synaptic clustering, active channels and reduction of membrane resistance by ongoing synaptic activity will have additional profound effects in vivo, the present in vitro modelling suggests that passive signal transmission in neurons will depend on type of signal conveyed, on directionality and on membrane state. This will be particularly important for thalamic interneurons, whose presynaptic dendrites may either work independently or function in concert with each other and with the soma. Our findings suggest that bursts may be particularly well transmitted along dendrites, allowing firing format to alter the functional anatomy of the cell.

Laminar and cellular targets of individual thalamic reticular nucleus axons in the lateral geniculate nucleus in the prosimian primate Galago.

The visual sector of the thalamic reticular nucleus is the source of the primary inhibitory projection to the visual thalamic relay nucleus, the dorsal lateral geniculate nucleus. The purpose of this study was to investigate laminar and cellular targets of individual thalamic reticular nucleus axons in the highly laminated lateral geniculate nucleus of the prosimian primate Galago to better understand the nature and function of this projection. Thalamic reticular axons labeled anterogradely by means of biotinylated dextran amine were examined by using light microscopic serial reconstruction and electron microscopic analysis in combination with postembedding immunohistochemical labeling for the neurotransmitter gamma-aminobutyric acid (GABA). The synaptic targets of labeled reticular terminal profiles were primarily GABA-negative dendrites (79-84%) of thalamocortical cells, whereas up to 16% were GABA-positive dendritic shafts or F2 terminals of interneurons. Reconstructed thalamic reticular nucleus axons were narrowly aligned along a single axis perpendicular to the geniculate laminar plane, exhibiting a high degree of visuotopic precision. Individual reticular axons targeted multiple or all geniculate laminae, with little laminar selectivity in the distribution of swellings with regard to the eye of origin or to the parvocellular, koniocellular, or magnocellular type neurons contained in the separate layers of the Galago lateral geniculate nucleus. These results suggest that cells in the visual thalamic reticular nucleus influence the lateral geniculate nucleus retinotopically, with little regard to visual functional streams.

Effects of activation of the histaminergic tuberomammillary nucleus on visual responses of neurons in the dorsal lateral geniculate nucleus.

We investigated the effects of the central histaminergic system on afferent sensory signals in the retinogeniculocortical pathway in the intact brain. Extracellular physiological recordings in vivo were obtained from neurons in the cat dorsal lateral geniculate nucleus (LGN) in conjunction with electrical activation of the histamine-containing cells in the tuberomammillary nucleus of the hypothalamus. Tuberomammillary activation resulted in a rapid and significant increase in the amplitude of baseline activity and visual responses in LGN neurons. Geniculate X- and Y-cells were affected similarly. LGN cells that exhibited a burst pattern of activity in the control condition switched to a tonic firing pattern during tuberomammillary activation. Effects on visual response properties were assessed using drifting sinusoidal gratings of varied spatial frequency. The resultant spatial tuning curves were elevated by tuberomammillary activation, but there was no change in tuning curve shape. Rather, the effect was proportionate to the control response, with the greatest tuberomammillary effects at spatial frequencies already optimal for the cell. Tuberomammillary activation caused a small phase lag in the visual response that was similar at all spatial frequencies, consistent with the induced shift from burst to tonic firing mode. These results indicate a significant histaminergic effect on LGN thalamocortical cells, with no clear effect on thalamic inhibitory neurons. The histaminergic system appears to strengthen central transmission of afferent information, intensifying but not transforming the retinally derived signals. Promoting sensory input may be one way in which the histaminergic system plays a role in arousal.