Thalamocortical Connections in the Pond Turtle <i>Pseudemys scripta elegans</i>

Abstract

Thalamocortical connections are a neuroanatomical feature shared among vertebrates, although the extent and organization of these connections vary among species. From an evolutionary standpoint, reptiles represent early stages of the pattern of connectivity between the thalamus and cortex, and elucidation of these pathways may help to reveal the biological significance of these projections. The present tract tracing study was performed to examine the organization of thalamocortical projections in the pond turtle, Pseudemys scripta elegans. All experiments were carried out using in vitro brain preparations. Injections of neurobiotin into the medial cortex resulted in labeled neurons in the ipsilateral dorsomedial anterior nucleus of the thalamus, those in the dorsomedial cortex labeled neurons in the dorsolateral anterior nucleus, and injections into the dorsal cortex resulted in labeled neurons in the dorsal lateral geniculate nucleus of the thalamus. Injections of neurobiotin into these thalamic nuclei confirmed the projections to the cortex. Finally, neurobiotin injections primarily into the medial cortex resulted in bilateral label of axons and terminals in the suprapeduncular nucleus of the hypothalamus. The results of the neurobiotin injections revealed a topographic pattern of thalamocortical connections such that medial cortical regions connect with medial thalamic nuclei and lateral cortical regions connect with lateral nuclei. These findings suggest that the presence of functionally segregated thalamocortical projections is a conserved feature of brain organization among amniotes. Moreover, this work describes a descending pathway linking cortical regions with the red nucleus via the hypothalamus thereby providing indirect cortical control of the reptilian rubrospinal system.

On the Value of Reptilian Brains to Map the Evolution of the Hippocampal Formation

“…However, while some aspects of cortical connections may be preserved, it is important to consider a wider range of cortical projections, since circuits may evolve independently, even in closely related species [Striedter, 1994]. In reptiles, M, DM, and dorsal cortex receive input from anterior thalamic nuclei [Bruce and Butler, 1984;Desan, 1988;Martinez-Garcia and Lorente, 1990;Hoogland et al, 1998;Desfilis et al, 2002;Zhu et al, 2005;Pritz, 2014], whereas mammals lack such input to the dentate gyrus and CA3. Thus, signals transmitted through the anterior thalamus in mammals, such as information about head direction [Taube, 2007], may reach a wider set of medial cortex regions in reptiles.…”

Putting the “Biology” Back into “Neurobiology”: The Strength of Diversity in Animal Model Systems for Neuroscience Research

“…Many conserved features of brain organization are observed throughout amniotes. For example, topographic analysis of thalamocortical projections using tract tracing in turtles showed a medial to lateral specificity of connections (Zhu et al, 2005). This finding indicates the presence of a functionally segregated pattern of thalamocortical projections that is a conserved feature of brain organization among amniotes.…”

Thalamo‐telencephalic pathways in the fire‐bellied toad Bombina orientalis

“…In turtles, three divisions of the anterior thalamus are recognized, viz., the dorsomedial anterior, dorsolateral anterior, and lateral geniculate nuclei, each of which targets different pallial territories following a medial to lateral pattern in the sense that the dorsomedial anterior nucleus projects to the medial cortex, the dorsolateral anterior nucleus to the dorsomedial cortex, and the lateral geniculate nucleus to the dorsal cortex (Ulinski, 1990;Zhu et al, 2005). Turtles also possess a collothalamic nucleus rotundus, which projects to the striatum as well as to the dorsal ventricular ridge (Hall and Ebner, 1970;Ulinski, 1990;Cordery and Molná r, 1999).…”

“…Are the medial subpallial and hypothalamic/tubercular projections of the anterior thalamus, which are seen in Bombina, characteristics shared by all tetrapods? Anterior thalamic projections to the medial subpallium have been described in a turtle (Siemen and Kü nzle, 1994;Zhu et al, 2005), and projections to the hypothalamus might also exist (Belekhova, 1991;Bruce and Neary, 1995b). In mammals, midline thalamic nuclei target the nucleus accumbens, septum, and prefrontal cortex, and evidence exists that single neurons target multiple sites (Otake and Nakamura, 1998;Van der Werf et al, 2002;Vertes et al, 2006).…”