Segregation of parallel inputs to the anteromedial and anteroventral thalamic nuclei of the rat

Abstract

Many brain structures project to both the anteroventral thalamic nucleus and the anteromedial thalamic nucleus. In the present study, pairs of different tracers were placed into these two thalamic sites in the same rats to determine the extent to which these nuclei receive segregated inputs. Only inputs from the laterodorsal tegmental nucleus, the principal extrinsic cholinergic source for these thalamic nuclei, showed a marked degree of collateralisation, with approximately 13% of all cells labelled in this tegmental area projecting to both nuclei. Elsewhere, double labelled cells were very scarce, comprising ~1% of all labelled cells. Three general patterns of anterior thalamic innervation were detected in these other areas. In some sites, e.g., prelimbic cortex, anterior cingulate cortex, and secondary motor area, cells projecting to the anteromedial and anteroventral thalamic nuclei were closely intermingled, with often only subtle distribution differences. These same projections were also often intermingled with inputs to the mediodorsal thalamic nucleus, but again there was little or no collaterisation. In other sites, e.g., the subiculum and retrosplenial cortex, there was often less overlap of cells projecting to the two anterior thalamic nuclei. A third pattern related to the dense inputs from the medial mammillary nucleus, where well defined topographies ensured little intermingling of the neurons that innervate the two thalamic nuclei. The finding, that a very small minority of cortical and limbic inputs bifurcate to innervate both anterior thalamic nuclei, highlights the potential for parallel information streams to control their functions, despite arising from common regions.

The subiculum

“…Only a small proportion of individual subiculum cells project to multiple sites (Naber and Witter, 1998;Namura et al, 1994;Wright et al, 2010Wright et al, , 2013. A possible exception concerns those subiculum projections reaching both the entorhinal cortex and septum (Donovan and Wyss, 1983; but see Naber and Witter, 1998).…”

Anterior thalamic nuclei lesions and recovery of function: Relevance to cognitive thalamus

“…This idea of an extended hippocampal system with the ATN as a nodal point in the exchange of both cortical and subcortical information relevant to episodic memory receives considerable support from anatomical descriptions of the neural connections of the ATN Dillingham et al, 2014;Jankowski et al, 2013;Wright et al, 2013). The three subnuclei of the ATN have a substantial number of reciprocal connections with the subicular cortex of the hippocampus and with the prefrontal cortex.…”

“…2). In addition, the apparent segregation of information transfer to the ATN, both directly from the hippocampal formation and from the MB, as well as from the prefrontal cortex, suggests the potential for subcircuit specificity (Jankowski et al, 2013;Wright et al, 2013).…”

Anterior nucleus of the thalamus

“…The anterodorsal is interconnected with the lateral mammillary nucleus, postsubiculm, and retrosplenial cortex, as a component of a circuit that conveys head-direction signals required for spatial navigation 1 (table). Many brains areas, including the anterior cingulate cortex, retrosplenial cortex, subiculum, and medial mammillary nucleus, project to both the AV and the AM; only a small minority of cortical and limbic inputs bifurcate to innervate both nuclei, which provides a potential for parallel information streams to control ANT function. 2 The ANT also receives subcortical inputs from the basolateral Figure 1 Main connections of the anterior nucleus of the thalamus Each subdivision of the anterior nucleus of thalamus (ANT) has a different pattern of connectivity and forms part of 3 different episodic memory subsystems involving the hippocampus, mammillary bodies, and neocortex. The anteromedial (AM) subnucleus has extensive reciprocal connections with the anterior cingulate and orbitomedial prefrontal cortex and relays information from the subiculum, retrosplenial cortex, and medial mammillary nucleus (MM) to the medial frontal lobe.…”

“…The connectivity patterns and electrophysiology of the ANT have been the subject of several reviews. [1][2][3][4] ORGANIZATION AND CONNECTIVITY OF THE ANTERIOR THALAMIC NUCLEUS Subdivisions, inputs, and outputs. The anterior nuclear group (ANT) occupies the superior region of the thalamus and is separated from the rest of the thalamus by the anterior internal medullary lamina, resulting in a Y-shaped lamina.…”