Retinal projections to the midline and intralaminar thalamic nuclei in the common marmoset (Callithrix jacchus).

In this study, we report the identification of a hitherto not reported direct retinal projection to midline and intralaminar thalamic nuclei in the marmoset brain. After unilateral intravitreal injections of cholera toxin subunit B (CTb), anterogradely transported CTb-immunoreactive fibers and presumptive terminals were seen in the following thalamic midline nuclei: paraventricular, rhomboid, interanteromedial, and reuniens, and thalamic intralaminar nuclei: central medial, central lateral, central dorsal, and parafascicular. Studies employing sensitive tracers in other primate species are needed in order to verify the possible universality of these projections. Some of the possible functional correlates of the present data are briefly discussed. The present results may contribute to the elucidation of the anatomical substrate of the functionally demonstrated involvement of this midline/intralaminar thalamic nuclear complex in several domains that include arousal and awareness, besides specific cognitive, sensory, and motor functions.

The visceral sector of the thalamic reticular nucleus in the rat.

Visceral sensory perception is subjected to modulation by attention or distraction, like other sensory systems. The thalamic reticular nucleus is a key region in selective attention, effecting a change in the mode of thalamocortical transmission. Each major thalamocortical system is connected with a particular sector of the thalamic reticular nucleus. No connections from the thalamic reticular nucleus have been described to the visceral sensory thalamus. We used axonal tracing techniques to study the possible existence of reciprocal connections between the visceral sensory relay in the lateral ventroposterior parvicellular thalamic nucleus, and the reticular nucleus of the thalamus. We also studied the projections from the visceral sensory cortex, located in the granular insular cortex in the rat, to the reticular nucleus of the thalamus. We found a convergent input from both thalamic and cortical sensory visceral regions to the same sector of the reticular nucleus of the thalamus. This visceral sector in turn sent GABAergic feedback connections to the lateral ventroposterior parvicellular thalamic nucleus. In addition, the visceral thalamus received histaminergic projections from the tuberomammillary nucleus, and noradrenergic projections from the locus coeruleus; both nuclei belong to the ascending activating system. Our findings indicate that the visceral sensory thalamocortical pathway is connected to the same subcortical structures that provide attention mechanisms for other thalamocortical systems.