The network organization of rat intrathalamic macroconnections and a comparison with other forebrain divisions Larry W. Swansona,1, Olaf Spornsb,c, and Joel D. Hahna aDepartment of Biological Sciences, University of Southern California, Los Angeles, CA 90089; bIndiana University Network Science Institute, Indiana University, Bloomington, IN 47405; and cDepartment of Psychological and Brain Sciences, Indiana University, Bloomington, IN 47405 Contributed by Larry W. Swanson, May 17, 2019 (sent for review April 8, 2019; reviewed by Zhanyan Fu and Leah A. Krubitzer) The thalamus is 1 of 4 major divisions of the forebrain and is of cortical regions (6). In contrast, the THe (habenular nuclei) and usually subdivided into epithalamus, dorsal thalamus, and ventral THv (reticular and ventral lateral geniculate nuclei, intergeniculate thalamus. The 39 gray matter regions comprising the large dorsal leaflet, zona incerta, and fields of Forel) are much smaller and thalamus project topographically to the cerebral cortex, whereas have virtually no projections to the cerebral cortex (6). the much smaller epithalamus (2 regions) and ventral thalamus (5 For the purposes of this analysis, axonal connections among all regions) characteristically project subcortically. Before analyzing 46 thalamic nuclei recognized on one side of the rat brain in a extrinsic inputs and outputs of the thalamus, here, the intrinsic standard atlas (7) have been considered (whether THe, THd, or connections among all 46 gray matter regions of the rat thalamus THv) along with the connections established by these nuclei with on each side of the brain were expertly collated and subjected to the 46 corresponding thalamic nuclei on the other side of the network analysis. Experimental axonal pathway-tracing evidence brain. Monosynaptic connection reports using axonal pathway- was found in the neuroanatomical literature for the presence or tracing methods were collated from the structural neuroscience absence of 99% of 2,070 possible ipsilateral connections and 97% of literature at the macroscale (“from gray matter region A to gray 2,116 possible contralateral connections; the connection density of matter region B”) level (8, 9), the only granularity level globally ipsilateral connections was 17%, and that of contralateral connec- represented thus far in the adult vertebrate literature. The NEUROSCIENCE tions 5%. One hub, the reticular thalamic nucleus (of the ventral resulting connection matrix was subjected to formal network thalamus), was found in this network, whereas no high-degree rich analysis. Collation was restricted to data from the rat where, by club or clear small-world features were detected. The reticular far, the most published connection data exist, and all such data thalamic nucleus was found to be primarily responsible for confer- ring the property of complete connectedness to the intrathalamic were converted, if necessary, to the only available nomenclature network in the sense that there is, at least, one path of finite length scheme that is formally defined, complete, internally consistent, between any 2 regions or nodes in the network. Direct comparison and hierarchically organized (7). with previous investigations using the same methodology shows The goals of this research are to create a gold-standard online that each division of the forebrain (cerebral cortex, cerebral nuclei, database of intrathalamic connections, to provide a top-level con- thalamus, hypothalamus) has distinct intrinsic network topological ceptual model for understanding intrathalamic circuitry at finer organization. A future goal is to analyze the network organization levels of granularity (neuron types within a region, individual of connections within and among these 4 divisions of the forebrain. Significance connectomics | mammal | neural connections | neuroinformatics | subsystems The thalamus is 1 of 4 major divisions of the forebrain, and one key function is to act as a “relay” for specific types of in- lassically, the forebrain has 4 divisions, the cerebral cortex formation to reach the cerebral cortex in an ordered way. A Cand cerebral nuclei (together the endbrain), and the thala- matrix of axonal connections among the 46 rat thalamic nuclei mus and hypothalamus—together the interbrain (1, 2). For the on each side of the brain was derived from collated data and systematic creation of a top-level network analysis of the mam- used to clarify the organization of this intrinsic thalamic cir- malian brain’s wiring diagram, we began rostrally with the in- cuitry with network analysis tools. Compared with the other 3 trinsic bilateral connectivity of the cerebral cortex (3), the forebrain divisions, the intrathalamic network is sparsely con- cerebral nuclei (4), and the hypothalamus (5). The results of this nected, and only one region, the reticular thalamic nucleus, is a type of analysis provide insight into how the network of associ- hub. Direct comparisons using the same network analysis tools ation (ipsilateral) and commissural (contralateral) connections indicate that each division of the forebrain has a distinct set of for a particular nervous system division are organized when intrinsic network organizational features. considered in isolation; that is, without accounting for axonal inputs from, and outputs to, other parts of the nervous system. Author contributions: L.W.S. designed research; L.W.S. performed research; L.W.S., O.S., and J.D.H. analyzed data; and L.W.S. wrote the paper with contributions from all authors. This study provides a similar analysis for the thalamus and the Reviewers: Z.F., Broad Institute of MIT and Harvard; and L.A.K., University of California, opportunity to compare its intrinsic circuitry with that described Davis. for each of the other 3 major divisions of the forebrain. The authors declare no conflict of interest. The thalamus is considered classically to have 3 major subdi- Published under the PNAS license. visions based on developmental, topographic, and extrinsic Data deposition: All connection reports used for this study are provided in a tabulated connectional patterns: epithalamus (THe), dorsal thalamus spreadsheet (Microsoft Excel file) in SI Materials and Methods; they are also deposited as a (THd), and ventral thalamus (THv) (6). The THd is by far the searchable resource at The Neurome Project, neuromeproject.org (https://sites.google. largest of these subdivisions in terms of its gray matter volume com/view/the-neurome-project/connections/thalamus?authuser=0). and the number of its gray matter regions (39 out of 46 for the 1To whom correspondence may be addressed. Email: [email protected]. complete thalamus on each side of the brain), and it is regarded as This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. the “gateway” to the cerebral cortex with each gray matter region 1073/pnas.1905961116/-/DCSupplemental. (or nucleus or node) projecting to a specific cortical region or set www.pnas.org/cgi/doi/10.1073/pnas.1905961116 PNAS Latest Articles | 1of9 Downloaded by guest on September 25, 2021 neurons within a neuron type, and synapses associated with individual to give 17,590 connection reports for 8,372 possible connections neurons) (10), and to compare the organization of intrathalamic arising from both sides of the brain). The connection reports were circuitry with the intrinsic circuitry of other major forebrain divi- from 24 journals (49.0% from the Journal of Comparative Neurology sions. Future goals are to analyze the organization of connections and 15.4% from Brain Research) involving about 68 laboratories. In among these divisions of the forebrain, and then their connections total, 15 different methods were used in generating the connection with the rest of the nervous system. reports; the pathway-tracing method and other metadata associated with each report are identified in Dataset S1. Results There are 2,070 (462−46) possible ipsilateral (uncrossed, asso- Basic Connection Numbers. The collation identified 347 ipsilateral ciation) macroconnections among the 46 gray matter regions of intrathalamic connections as present and 1,703 as absent for a the rat thalamus on one side of the brain (a connection from a connection density of 16.9% (347/2,050). In contrast, 112 con- region to itself is not considered) and 2,116 (462) possible con- tralateral connections from one thalamus to the other were tralateral (crossed, commissural) macroconnections from those identified as present, and 1,937 were identified as absent for a 46 regions to the corresponding regions of the thalamus on the connection density of 5.5% (112/2,049). No published data were other side of the brain. Thus, the thalamus on one side has found for 20 (1.0%) of all 2,070 possible ipsilateral connections 4,186 possible ipsilateral and contralateral connections, and the for a matrix coverage (fill ratio) of 99.0% (Fig. 1A), whereas right and left thalami together have 8,372 possible connections. matrix coverage for contralateral connections from one thalamus Our systematic review of the primary structural neuroscience to the other was 96.8% (no article found for 67, 3.2%, of all literature identified no reports of statistically significant male/ 2,116 possible connections). Assuming the connection reports female, right/left, or strain differences for any ipsilateral or collected from the literature representatively sample the 46- contralateral intrathalamic connections used for the analysis, region matrix for each