Organization of the rostral thalamus in the rat: Evidence for connections to layer I of visual cortex

Abstract

The present study demonstrates the organization of a thalamocortical projecting system which terminates within layer I of the visual cortex in the hooded rat. Horseradish peroxidase (HRP) injections restricted to layer I resulted in retrograde labeling of large and medium-sized multipolar and fusiform neurons that are located within the ventromedial (VM) nucleus and a dorsomedial subunit of the ventral anterolateral nucleus (VAL). Retrograde cellular labeling also occurs within the anteromedial nucleus (AM) following these injections. After restriction of HRP injections to layer I, peroxidase labeling was not found within neurons of the classically defined intralaminar system, i.e., central medial, paracentral, and central lateral nuclei, or within the rostral continuations of the intralaminar system. Since the VM, dorsomedial VAL, and AM nuclei are directly adjacent to portions of the internal medullary lamina, we refer to this amalgam of rostral thalamic nuclei that project to layer I as the "paralaminar" system. We also provide cytoarchitectonic criteria that can be used to distinguish three separate subdivisions within the VAL complex, including that portion of the VAL which is part of the "paralaminar" system. In contrast, when control injections of WGA-HRP are placed within either the cellular supragranular or infragranular layers of the visual cortex, no appreciable number of neurons are labeled within the VM, VAL, or AM.

Derivation and Analysis of Basic Computational Operations of Thalamocortical Circuits

“…The projection patterns described are thus arranged into two large thalamocortical circuits: 1) topographic projection of thalamic core cells > middle cortical layers > superficial layers > deep layers, with reciprocal topographic feedback from layer VI back to both the core cells and to the overlying portion of nucleus reticularis; 2) matrix cells projecting nontopographically to layer I and receiving projections back from layer V, without interposed nucleus reticularis projections. Evidence suggests that the repeating thalamocortical, cortico-cortical, and corticothalamic projection patterns hold not only for primary sensory areas including VPM/VPL, LGd, and MGv to layer IV, and Pom, LP/Pul, and MGm to layer I of somatosensory, visual and auditory cortices, respectively (Killackey and Ebner, 1972;Ryugo and Killackey, 1974;Ribak and Peters, 1975;Herkenham, 1980;Kelly and Wong, 1981;Swadlow, 1983;Rieck and Carey, 1985;Herkenham, 1986;Jensen and Killackey, 1987;Winer and Larue, 1987;Scheel, 1988;Conley and Diamond, 1990;Rouiller and Welker, 1991;Bourassa and Deschenes, 1995;Huang and Winer, 2000), but also for a wide array of thalamic nuclei, intralaminar and nonintralaminar alike (Jones and Hendry, 1989;Rausell et al, 1992;Molinari et al, 1994;Molinari et al, 1995;Kuroda et al, 1998;Mitchell and Cauller, 2001;Rauschecker et al, 1997;Jones, 1998;Reep and Corwin, 1999;Linke and Schwegler, 2000;Jones, 2001)). For those cortical areas not receiving topographic projections from thalamus, the extensive topography-preserving cortico-cortical projections from superficial layers to recipient middle layers with reciprocal projections from the target's deep layers back to the source's superficial layers, may subserve a related function …”

Nigrothalamic projections and nigrothalamocortical pathway to the medial agranular cortex in the rat: Single- and double-labeling light and electron microscopic studies

“…Despite numerous descriptions of the rat thalamus Mehler, 1976, 1985;Jones, 1985;Rieck and Carey, 1985;Paxinos and Watson, 1986;Sawyer et al, 1989;Deniau and Chevalier, 1992;Swanson, 1992;Price, 1995), there is no consistency in the delineation of the rat ventral thalamus. Therefore, for the most part, we have relied upon the thalamic parcellation presented in the atlas of Paxinos and Watson (1986).…”

“…On the basis of our own analysis of a Nissl-and AChE-stained series through the rat thalamus, we were unable to determine a boundary between the VA and the VL. Therefore, we have adopted the designation of this region as the VAL (Faull and Mehler, 1976;Rieck and Carey, 1985;Sawyer et al, 1989Sawyer et al, , 1991Sawyer et al, , 1994. We found that the primary nigral projection was to the rostromedial part of the VAL and VM.…”

“…In the rat, the motor thalamus is thought to consist of two components, the ventromedial nucleus (VM) and a larger rostrodorsal portion, which has been designated either the ventral lateral nucleus (VL; Faull and Mehler, 1985;Paxinos and Watson, 1986;Deniau and Chevalier, 1992;Price, 1995) or the ventral anterior-ventral lateral complex (VAL; Faull and Mehler, 1976;Rieck and Carey, 1985;Sawyer et al, 1989Sawyer et al, , 1991Sawyer et al, , 1994. There is a general consensus that the nigrothalamic projections to the rat motor thalamus are confined to the VM (Faull and Carman, 1968;Clavier et al, 1976;Faull and Mehler, 1978;Beckstead et al, 1979;Bentivoglio et al, 1979;DiChiara et al, 1979;Herkenham, 1979;Arbuthnott and Wright, 1982;Gerfen et al, 1982;Sawyer et al, 1989;Deniau and Chevalier, 1992).…”

Afferents to different layers of the dorsolateral isocortex in rats

“…Further work has demonstrated first, that not only the ventromedial nucleus, but also some other thalamic nuclei, project predominantly to the first layer. These nuclei are named "paralamellar" (Rieck and Carey 1985;Herkenham 1986). Secondly, it was found that the intralaminar nuclei project diffusely to the cortex, but only to deep layers (review in Herkenham 1986).…”