Limbic thalamus in rabbit: Architecture, projections to cingulate cortex and distribution of muscarinic acetylcholine, GABAA, and opioid receptors
Authors: Vogt Leslie J. , Vogt Brent A. , Sikes Robert W. 
Published: January 2005

Nuclei of the thalamus that project to cingulate cortex have been implicated in responses to noxious stimuli, cholinergic and motor functions. The rabbit limbic thalamus may play an important role in these functions, but has not been studied extensively in terms of its cytoarchitecture, the topographical organization of its cortical projections, and differential transmitter regulation of its subnuclei. Therefore, the architecture, projections to cingulate cortex, and radioligand binding were investigated in the anterior, ventral, lateral, and midline nuclei of rabbit thalamus. The anterior nuclei are highly differentiated because both the dorsal and ventral nuclei have parvicellular and magnocellular divisions. Fluorescent dyes were injected into cingulate cortex to evaluate limbic thalamocortical connections. The anterior medial, submedial, and parafascicular nuclei project primarily to anterior cingulate cortex, while they have small or no projections to posterior areas. The ventral anterior and ventral lateral nuclei have a significant projection to dorsal cingulate cortex, including areas 24b and 29d. Projections of the anterior ventral nucleus are topographically organized, since medial parts of the parvicellular division project to rostral area 29, and lateral parts project to caudal area 29. The lateral nuclei and the parvicellular and magnocellular divisions of the anterior dorsal nucleus project with progressively higher densities in the rostrocaudal plane of area 29. Finally, the magnocellular division of the anterior ventral nucleus projects almost exclusively to caudal and ventral area 29, i.e., granular retrosplenial cortex. Ligand binding studies employed coverslip autoradiography and single grain counting techniques. Muscarinic receptor binding was moderate for both pirenzepine and oxotremorine‐M in the parvicellular anterior ventral nucleus, while in other nuclei, there was an inverse relationship in the binding for these ligands. Most notably, the anterior dorsal nucleus, which receives no cholinergic input, had very high oxotremorine‐M and low pirenzepine binding, while the anterior medial nucleus, which receives a moderate cholinergic input, had the highest pirenzepine binding and very low oxotremorine‐M binding. Muscimol binding to GABAA receptors was highest in the anterior ventral nucleus, while it was at moderate levels in the anterior dorsal and lateral nuclei. The binding of Tyr‐D‐Ala‐Gly‐MePhe‐Gly‐ol to mu opioid receptors and 2‐D‐penicillamine‐5‐D‐penicillamine‐enkephalin to delta opioid receptors were both high in the parvicellular and low in the magnocellular divisions of the anterior dorsal nucleus. The magnocellular division of the anterior ventral, the lateral dorsal, and the parafascicular nuclei had high mu opioid binding, while the lateral dorsal and lateral magnocellular nuclei had low levels of delta opioid binding. The submedial nucleus had moderate binding of both opioid compounds. These results suggest that the limbic thalamus in the rabbit is highly differentiated and has topographically organized connections to cingulate cortex. Some of these connections may mediate responses to noxious stimuli including those from the submedial and parafascicular nuclei. Finally, the ligand binding studies indicate that transmitter systems differentially regulate the various subnuclei within the limbic thalamus and may modulate different components of limbic thalamocortical circuitry. © 1992 Wiley‐Liss, Inc.