Morphological correlates of triadic circuitry in the lateral geniculate nucleus of cats and rats

Paired Feed-Forward Excitation With Delayed Inhibition Allows High Frequency Computations Across Brain Regions

The transmission of high frequency temporal information across brain regions is critical to perception, but the mechanisms underlying such transmission remain unclear. Long-range projection patterns across brain areas are often comprised of paired feed-forward excitation followed closely by delayed inhibition, including the thalamic triad synapse, thalamic projections to cortex, and projections within the hippocampus. Previous studies have shown that these joint projections produce a shortened period of depolarization, sharpening the timing window over which the postsynaptic neuron can fire. Here we show that these projections can facilitate the transmission of high frequency computations even at frequencies that are highly filtered by neuronal membranes. This temporal facilitation occurred over a range of synaptic parameter values, including variations in synaptic strength, synaptic time constants, short-term synaptic depression, and the delay between excitation and inhibition. Further, these projections can coordinate computations across multiple network levels, even amid ongoing local activity. We suggest that paired feed-forward excitation and inhibition provide a hybrid signal—carrying both a value and a clock-like trigger—to allow circuits to be responsive to input whenever it arrives.

Immunocytochemical and ultrastructural organization of the taste thalamus of the tree shrew (Tupaia belangeri)

Ventroposterior medialis parvocellularis (VPMP) nucleus of the primate thalamus receives direct input from the nucleus of the solitary tract, whereas the homologous thalamic structure in the rodent does not. To reveal whether the synaptic circuitries in these nuclei lend evidence for conservation of design principles in the taste thalamus across species or across sensory thalamus in general, we characterized the ultrastructural and molecular properties of the VPMP in a close relative of primates, the tree shrew (Tupaia belangeri), and compared these to known properties of the taste thalamus in rodent, and the visual thalamus in mammals. Electron microscopy analysis to categorize the synaptic inputs in the VPMP revealed that the largest-size terminals contained many vesicles and formed large synaptic zones with thick postsynaptic density on multiple, medium-caliber dendrite segments. Some formed triads within glomerular arrangements. Smaller-sized terminals contained dark mitochondria; most formed a single asymmetric or symmetric synapse on small-diameter dendrites. Immuno-EM experiments revealed that the large-size terminals contained VGLUT2, whereas the small-size terminal populations contained VGLUT1 or ChAT. These findings provide evidence that the morphological and molecular characteristics of synaptic circuitry in the tree shrew VPMP are similar to that in nonchemical sensory thalamic nuclei. Furthermore, the results indicate that all primary sensory nuclei of the thalamus in higher mammals share a structural template for processing thalamocortical sensory information. In contrast, substantial morphological and molecular differences in rodent versus tree shrew taste nuclei suggest a fundamental divergence in cellular processing mechanisms of taste input in these two species.

An Individual Interneuron Participates in Many Kinds of Inhibition and Innervates Much of the Mouse Visual Thalamus

One way to assess a neuron's function is to describe all its inputs and outputs. With this goal in mind, we used serial section electron microscopy to map 899 synaptic inputs and 623 outputs in one inhibitory interneuron in a large volume of the mouse visual thalamus. This neuron innervated 256 thalamocortical cells spread across functionally distinct subregions of the visual thalamus. All but one of its neurites were bifunctional, innervating thalamocortical and local interneurons while also receiving synapses from the retina. We observed a wide variety of local synaptic motifs. While this neuron innervated many cells weakly, with single en passant synapses, it also deployed specialized branches that climbed along other dendrites to form strong multi-synaptic connections with a subset of partners. This neuron's diverse range of synaptic relationships allows it to participate in a mix of global and local processing but defies assigning it a single circuit function.

Convergent synaptic inputs to layer 1 cells of mouse cortex

We used whole cell recordings from slice preparations of mouse cortex to identify various inputs to neurons of layer 1. Two sensory cortical areas were targeted: a primary somatosensory area, namely, the barrel cortex of S1, and a higher order visual area, namely, V2M. Results were similar from both areas. By activating local inputs using photostimulation with caged glutamate, we also identified glutamatergic (and possibly GABAergic) inputs from all lower layers plus GABAergic inputs from nearby layer 1 neurons. However, the patterns of such inputs to layer 1 neurons showed great variation among cells. In separate experiments, we found that electrical stimulation of axons running parallel to the cortical surface in layer 1 also evoked a variety of convergent input types to layer 1 neurons, including glutamatergic "drivers" and "modulators" plus classic modulatory inputs, including serotonergic, nicotinic, α- and β-adrenergic, from subcortical sites. Given that these layer 1 cells significantly affect the responses of other cortical neurons, especially via affecting the apical dendrites of pyramidal cells so important to cortical functioning, their role in cortical processing is significant. We believe that the data presented here lead to better understanding of the functioning of layer 1 neurons in their role of influencing cortical processing.

Synaptic Contributions to Receptive Field Structure and Response Properties in the Rodent Lateral Geniculate Nucleus of the Thalamus

Comparative physiological and anatomical studies have greatly advanced our understanding of sensory systems. Many lines of evidence show that the murine lateral geniculate nucleus (LGN) has unique attributes, compared with other species such as cat and monkey. For example, in rodent, thalamic receptive field structure is markedly diverse, and many cells are sensitive to stimulus orientation and direction. To explore shared and different strategies of synaptic integration across species, we made whole-cell recordings in vivo from the murine LGN during the presentation of visual stimuli, analyzed the results with different computational approaches, and compared our findings with those from cat. As for carnivores, murine cells with classical center-surround receptive fields had a "push-pull" structure of excitation and inhibition within a given On or Off subregion. These cells compose the largest single population in the murine LGN (∼40%), indicating that push-pull is key in the form vision pathway across species. For two cell types with overlapping On and Off responses, which recalled either W3 or suppressed-by-contrast ganglion cells in murine retina, inhibition took a different form and was most pronounced for spatially extensive stimuli. Other On-Off cells were selective for stimulus orientation and direction. In these cases, retinal inputs were tuned and, for oriented cells, the second-order subunit of the receptive field predicted the preferred angle. By contrast, suppression was not tuned and appeared to sharpen stimulus selectivity. Together, our results provide new perspectives on the role of excitation and inhibition in retinothalamic processing.

SIGNIFICANCE STATEMENT

We explored the murine lateral geniculate nucleus from a comparative physiological perspective. In cat, most retinal cells have center-surround receptive fields and push-pull excitation and inhibition, including neurons with the smallest (highest acuity) receptive fields. The same is true for thalamic relay cells. In mouse retina, the most numerous cell type has the smallest receptive fields but lacks push-pull. The most common receptive field in rodent thalamus, however, is center-surround with push-pull. Thus, receptive field structure supersedes size per se for form vision. Further, for many orientation-selective cells, the second-order component of the receptive field aligned with stimulus preference, whereas suppression was untuned. Thus, inhibition may improve spatial resolution and sharpen other forms of selectivity in rodent lateral geniculate nucleus.

Functional topographic organization of the motor reticulothalamic pathway

The thalamic reticular nucleus (TRN) is a thin layer of GABAergic cells lying rostral and lateral to the dorsal thalamus, and its projection to thalamic relay cells (i.e., the reticulothalamic pathway) strongly inhibits these cells. In an attempt to extend earlier studies of reticulothalamic connections to sensory thalamic nuclei, we used laser-scanning photostimulation to study the reticulothalamic projections to the main motor thalamic relays, the ventral anterior and lateral (VA and VL) nuclei, as well as to the nearby central lateral (CL) thalamic nucleus. VA/VL and the earlier studied somatosensory thalamic nuclei are considered "core" nuclei with topographic thalamocortical projections, whereas CL is thought to be a "matrix" nucleus with diffuse thalamocortical projections. We found that the TRN input footprints to VA/VL and CL are spatially localized and topographic and generally conform to the patterns established earlier for the TRN projections to sensory thalamic relays. These remarkable similarities suggest similar organization of reticulothalamic pathways and TRN regulation of thalamocortical communication for motor and sensory systems and perhaps also for core and matrix thalamus. Furthermore, we found that VA/VL and CL shared overlapping TRN input regions, suggesting that CL may also be involved in the relay of motor information.

Modulatory effects of activation of metabotropic glutamate receptors on GABAergic circuits in the mouse thalamus

Metabotropic glutamate receptors (mGluRs) are widely distributed in the central nervous system and modulate the release of neurotransmitters in different ways. We have previously shown that activation of presynaptic group II mGluRs reduces the gain of GABAergic inputs in both primary visual and auditory cortices (V1 and A1). In the present study, we sought to determine whether activation of mGluRs can also affect the inhibitory inputs in thalamus. Using whole cell recordings in a mouse slice preparation, we studied two GABAergic inputs to thalamic relay cells: that of the thalamic reticular nucleus (TRN) to cells of the ventral posteromedial nucleus (VPM) and that of interneurons to cells of the lateral geniculate nucleus (LGN). We found that activation of mGluRs significantly reduced the amplitudes of inhibitory postsynaptic currents (IPSCs) evoked from TRN inputs to VPM cells, and further experiments indicated that this was due to activation of presynaptic group I and group II mGluRs. Similar results were found in the interneuronal inputs to LGN cells. Activation of presynaptic group I (type 1 but not type 5) and group II mGluRs significantly reduced the amplitudes of evoked IPSCs of the axonal inputs to relay cells, and additional experiments were consistent with previous observations that activation of type 5 mGluRs on the dendritic terminals of interneurons enhanced postsynaptic IPSCs. We concluded that group I and II mGluRs may generally reduce the amplitude of evoked GABAergic IPSCs of axonal inputs to thalamic relay cells, operating through presynaptic mechanisms, and this extends our previous findings in cortex.

Thalamic microcircuits: presynaptic dendrites form two feedforward inhibitory pathways in thalamus

In the visual thalamus, retinal afferents activate both local interneurons and excitatory thalamocortical relay neurons, leading to robust feedforward inhibition that regulates the transmission of sensory information from retina to neocortex. Peculiarly, this feedforward inhibitory pathway is dominated by presynaptic dendrites. Previous work has shown that the output of dendritic terminals of interneurons, also known as F2 terminals, are regulated by both ionotropic and metabotropic glutamate receptors. However, it is unclear whether both classes of glutamate receptors regulate output from the same or distinct dendritic terminals. Here, we used focal glutamate uncaging and whole cell recordings to reveal two types of F2 responses in rat visual thalamus. The first response, which we are calling a Type-A response, was mediated exclusively by ionotropic glutamate receptors (i.e., AMPA and NMDA). In contrast, the second response, which we are calling a Type-B response, was mediated by a combination of ionotropic and type 5 metabotropic glutamate receptors (i.e., mGluR(5)). In addition, we demonstrate that both F2 responses are evoked in the same postsynaptic neurons, which are morphologically distinct from neurons in which no F2 responses are observed. Since photostimulation was relatively focal and small in magnitude, these results suggest distinct F2 terminals, or small clusters of terminals, could be responsible for generating the two inhibitory responses observed. Because of the nature of ionotropic and metabotropic glutamate receptors, we predict the efficacy by which the retina communicates with the thalamus would be strongly regulated by 1) the activity level of a given retinogeniculate axon, and 2) the specific type of F2 terminals activated.

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.

Spatially distinct actions of metabotropic glutamate receptor activation in dorsal lateral geniculate nucleus

Thalamocortical neurons in the dorsal lateral geniculate nucleus (dLGN) dynamically communicate visual information from the retina to the neocortex, and this process can be modulated via activation of metabotropic glutamate receptors (mGluRs). Neurons within dLGN express different mGluR subtypes associated with distinct afferent synaptic pathways; however, the physiological function of this organization is unclear. We report that the activation of mGluR(5), which are located on presynaptic dendrites of local interneurons, increases GABA output that in turn produces an increased inhibitory activity on proximal but not distal dendrites of dLGN thalamocortical neurons. In contrast, mGluR(1) activation produces strong membrane depolarization in thalamocortical neurons regardless of distal or proximal dendritic locations. These findings provide physiological evidence that mGluR(1) appear to be distributed along the thalamocortical neuron dendrites, whereas mGluR(5)-dependent action occurs on the proximal dendrites/soma of thalamocortical neurons. The differential distribution and activation of mGluR subtypes on interneurons and thalamocortical neurons may serve to shape excitatory synaptic integration and thereby regulate information gating through the thalamus.

Functional organization of the thalamic input to the thalamic reticular nucleus

Most axons connecting the thalamus and cortex in both directions pass through the thalamic reticular nucleus (TRN), a thin layer of GABAergic cells adjacent to the thalamus, and innervate neurons there. The TRN, therefore, is in a strategic location to regulate thalamocortical communication. We recorded neurons of the somatosensory region of the TRN in a thalamocortical slice preparation and studied the spatial organization of their thalamic input using laser scanning photostimulation. We show that the thalamoreticular pathway is organized topographically for most neurons. The somatosensory region of the TRN can be organized into three tiers. From the inner (thalamoreticular) border to the outer, in a manner roughly reciprocal to the reticulothalamic pathway, each of these tiers receives its input from one of the somatosensory relays of the thalamus--the posterior medial, ventroposterior medial, and ventroposterior lateral nuclei. What is surprising is that approximately a quarter of the recorded neurons received input from multiple thalamic regions usually located in different nuclei. These neurons distribute evenly throughout the thickness of the TRN. Our results, therefore, suggest that there exist a subpopulation of TRN neurons that receive convergent inputs from multiple thalamic sources and engage in more complex patterns of inhibition of relay cells. We propose these neurons enable the TRN to act as an externally driven "searchlight" that integrates cortical and subcortical inputs and then inhibits or disinhibits specific thalamic relay cells, so that appropriate information can get through the thalamus to the cortex.

Functional organization of the somatosensory cortical layer 6 feedback to the thalamus

The pathway from cortical layer 6 to the thalamus is a property of all thalamic relay nuclei. This pathway, as a population, directly excites relay cells and indirectly inhibits them via the thalamic reticular nucleus. To understand the circuit organization of this cortical feedback, we used laser-scanning photostimulation, which specifically activates somata or dendrites, to stimulate the primary somatosensory cortex in an in vitro thalamocortical slice preparation while recording from neurons of the ventral posterior medial nucleus. Layer 6 photostimulation evoked biphasic excitatory postsynaptic current/inhibitory postsynaptic current (EPSC/IPSC) responses in the neurons of the ventral posterior medial nucleus, indicating that such photostimulation strongly activates reticular cells. These disynaptic IPSCs were greatly suppressed or abolished by bath application of the muscarinic agonist acetyl-beta-methylcholine. Our results suggest that the top-down modulation of thalamic neurons from cortical layer 6 involves an inhibitory component via the thalamic reticular nucleus, and this component can be selectively reduced by cholinergic input. Finally, we found the footprints for the excitatory corticothalamic and the inhibitory cortico-reticulo-thalamic inputs to be located in similar positions, though in some cases they are offset. Both patterns have implications for cortico-reticulo-thalamic circuitry.

Different topography of the reticulothalmic inputs to first- and higher-order somatosensory thalamic relays revealed using photostimulation

The thalamic reticular nucleus is a layer of GABAergic neurons that occupy a strategic position between the thalamus and cortex. Here we used laser scanning photostimulation to compare in young mice (9-12 days old) the organization of the reticular inputs to first- and higher-order somatosensory relays, namely, the ventral posterior lateral nucleus and posterior nucleus, respectively. The reticulothalamic input footprints to the ventral posterior lateral nucleus neurons consisted of small, single, topographically organized elliptical regions in a tier away from the reticulothalamic border. In contrast, those to the posterior nucleus were complicated and varied considerably among neurons: although almost all contained a single elliptical region near the reticulothalamic border, in most cases, they consisted of additional discontinuous regions or relatively diffuse regions throughout the thickness of the thalamic reticular nucleus. Our results suggest two sources of reticular inputs to the posterior nucleus neurons: one that is relatively topographic from regions near the reticulothalamic border and one that is relatively diffuse and convergent from most or all of the thickness of the thalamic reticular nucleus. We propose that the more topographic reticular input is the basis of local inhibition seen in posterior nucleus neurons and that the more diffuse and convergent input may represent circuitry through which the ventral posterior lateral and posterior nuclei interact.

Synaptic release generates a tonic GABA(A) receptor-mediated conductance that modulates burst precision in thalamic relay neurons

Tonic inhibition has emerged as a key regulator of neuronal excitability in the CNS. Thalamic relay neurons of the dorsal lateral geniculate nucleus (dLGN) exhibit a tonic GABA(A) receptor (GABA(A)R)-mediated conductance that is correlated with delta-subunit expression. Indeed, consistent with the absence of delta-subunit expression, no tonic conductance is found in the adjacent ventral LGN. We show that, in contrast to the situation in cerebellar granule cells, thalamic delta-subunit-containing GABA(A)Rs (delta-GABA(A)Rs) do not contribute to a spillover component of IPSCs in dLGN. However, tonic activation of thalamic delta-GABA(A)Rs is sensitive to the global level of inhibition, showing an absolute requirement on the synaptic release of GABA. Thus, the tonic conductance is abolished when transmitter release probability is reduced or action potential-evoked release is blocked. We further show that continuous activation of delta-GABA(A)Rs introduces variability into the timing of low-threshold rebound bursts. Hence, activation of delta-GABA(A)Rs could act to destabilize thalamocortical oscillations and therefore have an important impact on behavioral state.

Mapping of the Functional Interconnections Between Thalamic Reticular Neurons Using Photostimulation

The thalamic reticular nucleus is strategically located in the axonal pathways between thalamus and cortex, and reticular cells exert strong, topographic inhibition on thalamic relay cells. Although evidence exists that reticular neurons are interconnected through conventional and electrical synapses, the spatial extent and relative strength of these synapses are unclear. To address these issues, we used uncaging of glutamate by laser-scanning photostimulation to provide precisely localized and consistent activation of reticular cell bodies and dendrites in an in vitro slice preparation from the rat as a means to study reticulo-reticular connections. Among the 47 recorded reticular neurons, 29 (62%) received GABAergic axodendritic input from an area immediately surrounding each of the recorded cell bodies, and 8 (17%) responded with depolarizing spikelets, suggesting inputs through electrical synapses. We also found that TTX completely blocked all evoked IPSCs, implying that any dendrodendritic synapses between reticular cells either are relatively weak, have no nearby glutamatergic receptors, or are dependent on back-propagation of action potentials. Finally, we showed that the GABAergic connections between reticular cells are weaker than those from reticular cells to relay cells. Our results suggest that the GABAergic axodendritic synapse is the dominant form of reticulo-reticular connectivity, and because they are much weaker than the reticulo-relay cell synapses, their functional purpose may be to regulate the spatial extent of the reticular inhibition on relay cells.

GABAA receptor-mediated tonic inhibition in thalamic neurons

Tonic GABAA receptor-mediated inhibition is typically generated by delta subunit-containing extrasynaptic receptors. Because the delta subunit is highly expressed in the thalamus, we tested whether thalamocortical (TC) neurons of the dorsal lateral geniculate nucleus (dLGN) and ventrobasal complex exhibit tonic inhibition. Focal application of gabazine (GBZ) (50 microM) revealed the presence of a 20 pA tonic current in 75 and 63% of TC neurons from both nuclei, respectively. No tonic current was observed in GABAergic neurons of the nucleus reticularis thalami (NRT). Bath application of 1 microM GABA increased tonic current amplitude to approximately 70 pA in 100% of TC neurons, but it was still not observed in NRT neurons. In dLGN TC neurons, the tonic current was sensitive to low concentrations of the delta subunit-specific receptor agonists allotetrahydrodeoxycorticosterone (100 nM) and 4,5,6,7-tetrahydroisoxazolo[5,4-c]-pyridin-3-ol (THIP) (100 nM) but insensitive to the benzodiazepine flurazepam (5 microM). Bath application of low concentrations of GBZ (25-200 nM) preferentially blocked the tonic current, whereas phasic synaptic inhibition was primarily maintained. Under intracellular current-clamp conditions, the preferential block of the tonic current with GBZ led to a small depolarization and increase in input resistance. Using extracellular single-unit recordings, block of the tonic current caused the cessation of low-threshold burst firing and promoted tonic firing. Enhancement of the tonic current by THIP hyperpolarized TC neurons and promoted burst firing. Thus, tonic current in TC neurons generates an inhibitory tone. Its modulation contributes to the shift between different firing modes, promotes the transition between different behavioral states, and predisposes to absence seizures.

Mapping by Laser Photostimulation of Connections Between the Thalamic Reticular and Ventral Posterior Lateral Nuclei in the Rat

We used laser scanning photostimulation through a focused UV laser of caged glutamate in an in vitro slice preparation through the rat’s somatosensory thalamus to study topography and connectivity between the thalamic reticular nucleus and ventral posterior lateral nucleus. This enabled us to focally stimulate the soma or dendrites of reticular neurons. We were thus able to confirm and extend previous observations based mainly on neuroanatomical pathway tracing techniques: the projections from the thalamic reticular nucleus to the ventral posterior lateral nucleus have precise topography. The reticular zone, which we refer to as a “footprint,” within which photostimulation evoked inhibitory postsynaptic currents (IPSCs) in relay cells, was relatively small and oval, with the long axis being parallel to the border between the thalamic reticular nucleus and ventral posterior lateral nucleus. These evoked IPSCs were large, and by using appropriate GABA antagonists, we were able to show both GABAA and GABAB components. This suggests that photostimulation strongly activated reticular neurons. Finally, we were able to activate a disynaptic relay cell-to-reticular-to- relay cell pathway by evoking IPSCs in relay cells from photostimulation of the region surrounding a recorded relay cell. This, too, suggests strong responses of relay cells, responses strong enough to evoke spiking in their postsynaptic reticular targets. The regions of photostimulation for these disynaptic responses were much larger than the above-mentioned reticular footprints, and this suggests that reticulothalamic axon arbors are less widespread than thalamoreticular arbors, that there is more convergence in thalamoreticular connections than in reticulothalamic connections, or both.