Most sensory information destined for the neocortex is relayed through the thalamus, where considerable transformation occurs 1 , 2 . One powerful means of transformation involves interactions between excitatory thalamocortical neurons that carry data to cortex and inhibitory neurons of the thalamic reticular nucleus (TRN) that regulate flow of those data 3 – 6 . Despite enduring recognition of its importance 7 – 9 , understanding of TRN cell types, their organization, and their functional properties has lagged that of the thalamocortical systems they control. Here we address this, investigating somatosensory and visual circuits of the TRN. In the somatosensory TRN we observed two groups of genetically defined neurons that are topographically segregated, physiologically distinct, and connect reciprocally with independent thalamocortical nuclei via dynamically divergent synapses. Calbindin-expressing cells, located in the central core, connect with the ventral posterior nucleus (VP), the primary somatosensory thalamocortical relay. In contrast, somatostatin-expressing cells, residing along the surrounding edges of TRN, synapse with the posterior medial thalamic nucleus (POM), a higher-order structure that carries both top-down and bottom-up information 10 – 12 . The two TRN cell groups process their inputs in pathway-specific ways. Synapses from VP to central TRN cells transmit rapid excitatory currents that depress deeply during repetitive activity, driving phasic spike output. Synapses from POM to edge TRN cells evoke slower, less depressing excitatory currents that drive more persistent spiking. Differences in intrinsic physiology of TRN cell types, including state-dependent bursting, contribute to these output dynamics. Thus, processing specializations of two somatosensory TRN subcircuits appear to be tuned to the signals they carry—a primary central subcircuit to discrete sensory events, and a higher-order edge subcircuit to temporally distributed signals integrated from multiple sources. The structure and function of visual TRN subcircuits closely resemble those of the somatosensory TRN. These results provide fundamental insights about how subnetworks of TRN neurons may differentially process distinct classes of thalamic information.
SUMMARY The induction of limb repair in adult vertebrates is a pressing, unsolved problem. Here, we characterize the effects of an integrated device that delivers drugs to severed hindlimbs of adult Xenopus laevis, which normally regenerate cartilaginous spikes after amputation. A wearable bioreactor containing a silk protein-based hydrogel that delivered progesterone to the wound site immediately after hindlimb amputation for only 24 hr induced the regeneration of paddle-like structures in adult frogs. Molecular markers, morphometric analysis, X-ray imaging, immunofluorescence, and behavioral assays were used to characterize the differences between the paddle-like structures of successful regenerates and hypomorphic spikes that grew in untreated animals. Our experiments establish a model for testing therapeutic cocktails in vertebrate hindlimb regeneration, identify pro-regenerative activities of progesterone-containing bioreactors, and provide proof of principle of brief use of integrated device-based delivery of small-molecule drugs as a viable strategy to induce and maintain a long-term regenerative response.
Most sensory information destined for the neocortex is relayed there from the thalamus, where considerable transformation occurs 1,2 . One powerful means of transformation involves interactions between excitatory thalamocortical neurons that carry data to cortex and inhibitory neurons of the thalamic reticular nucleus (TRN) that regulate flow of those data 3-6 . Despite enduring recognition of the TRN's importance 7-9 , understanding of its cell types, their organization, and their functional properties has lagged that of the thalamocortical systems it controls.Here we address this, investigating somatosensory and visual circuits of the TRN. First, in the somatosensory TRN we observed two groups of genetically defined neurons that are topographically segregated, physiologically distinct, and connect reciprocally with independent thalamocortical nuclei via dynamically divergent synapses. Calbindin-expressing cells, located in the central core, connect with the ventral posterior nucleus (VP), the primary somatosensory thalamocortical relay. In contrast, somatostatin-expressing cells, residing along the surrounding edges of TRN, synapse with the posterior medial thalamic nucleus (POM), a higher-order structure that carries both top-down and bottom-up information 10-12 . The two TRN cell groups process their inputs in pathway-specific ways. Synapses from VP to central TRN cells evoke rapid currents that depress deeply during repetitive activity, driving phasic spike output. In contrast, synapses from POM to edge TRN cells evoke slower, sustained currents that drive more persistent spiking. Differences in intrinsic physiology of TRN cell types, including state-dependent bursting, also contribute to these output dynamics. Thus, processing specializations of the two TRN circuits appear to be tuned to the types of signals they carry-the primary central circuit to discrete sensory events, and the higher-order edge circuit to temporally distributed signals integrated from multiple sources. Central and edge circuits of the visual TRN and their associated thalamocortical nuclei closely resemble those of the somatosensory circuits. These results provide fundamental insights about how subnetworks of TRN neurons could differentially process distinct classes of thalamic information.
Zaltsman JB, Heimel JA, Van Hooser SD. Weak orientation and direction selectivity in lateral geniculate nucleus representing central vision in the gray squirrel Sciurus carolinensis. J Neurophysiol 113: 2987-2997, 2015. First published February 25, 2015 doi:10.1152/jn.00516.2014.-Classic studies of lateral geniculate nucleus (LGN) and visual cortex (V1) in carnivores and primates have found that a majority of neurons in LGN exhibit a center-surround organization, while V1 neurons exhibit strong orientation selectivity and, in many species, direction selectivity. Recent work in the mouse and the monkey has discovered previously unknown classes of orientation-and direction-selective neurons in LGN. Furthermore, some recent studies in the mouse report that many LGN cells exhibit pronounced orientation biases that are of comparable strength to the subthreshold inputs to V1 neurons. These results raise the possibility that, in rodents, orientation biases of individual LGN cells make a substantial contribution to cortical orientation selectivity. Alternatively, the size and contribution of orientation-or direction-selective channels from LGN to V1 may vary across mammals. To address this question, we examined orientation and direction selectivity in LGN and V1 neurons of a highly visual diurnal rodent: the gray squirrel. In the representation of central vision, only a few LGN neurons exhibited strong orientation or direction selectivity. Across the population, LGN neurons showed weak orientation biases and were much less selective for orientation compared with V1 neurons. Although direction selectivity was weak overall, LGN layers 3abc, which contain neurons that express calbindin, exhibited elevated direction selectivity index values compared with LGN layers 1 and 2. These results suggest that, for central visual fields, the contribution of orientation-and directionselective channels from the LGN to V1 is small in the squirrel. As in other mammals, this small contribution is elevated in the calbindinpositive layers of the LGN motion; thalamocortical; thalamus; striate cortex; area 17; lateral geniculate body; lateral geniculate bodies
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