The dorsal tectal longitudinal column (TLCd): a second longitudinal column in the paramedian region of the midbrain tectum

M.-Auxiliadora Aparicio,Enrique Saldaña

doi:10.1007/s00429-013-0522-x

Abstract

The tectal longitudinal column (TLC) is a longitudinally oriented, long and narrow nucleus that spans the paramedian region of the midbrain tectum of a large variety of mammals (Saldaña et al. in J Neurosci 27:13108–13116, 2007). Recent analysis of the organization of this region revealed another novel nucleus located immediately dorsal, and parallel, to the TLC. Because the name “tectal longitudinal column” also seems appropriate for this novel nucleus, we suggest the TLC described in 2007 be renamed the “ventral tectal longitudinal column (TLCv)”, and the newly discovered nucleus termed the “dorsal tectal longitudinal column (TLCd)”. This work represents the first characterization of the rat TLCd. A constellation of anatomical techniques was used to demonstrate that the TLCd differs from its surrounding structures (TLCv and superior colliculus) cytoarchitecturally, myeloarchitecturally, neurochemically and hodologically. The distinct expression of vesicular amino acid transporters suggests that TLCd neurons are GABAergic. The TLCd receives major projections from various areas of the cerebral cortex (secondary visual mediomedial area, and granular and dysgranular retrosplenial cortices) and from the medial pretectal nucleus. It densely innervates the ipsilateral lateral posterior and laterodorsal nuclei of the thalamus. Thus, the TLCd is connected with vision-related neural centers. The TLCd may be unique as it constitutes the only known nucleus made of GABAergic neurons dedicated to providing massive inhibition to higher order thalamic nuclei of a specific sensory modality.

Highlights

The classical view of the midbrain tectum as being exclusively composed of the inferior and superior colliculi (IC and SC, respectively) is challenged by the recent discovery of the tectal longitudinal column (TLC; Saldana et al 2007)
The tectal longitudinal column (TLC) is a longitudinally oriented, long and narrow nucleus that spans the paramedian region of the midbrain tectum of a large variety of mammals (Saldana et al in J Neurosci 27:13108–13116, 2007)
For the sake of consistency, we suggest ascribing the name ‘‘ventral tectal longitudinal column (TLCv)’’ to the nucleus described by Saldana et al (2007), and ‘‘dorsal tectal longitudinal column (TLCd)’’ to the nucleus previously referred to as dorsal column (Table 1)

Summary

Introduction

The classical view of the midbrain tectum as being exclusively composed of the inferior and superior colliculi (IC and SC, respectively) is challenged by the recent discovery of the tectal longitudinal column (TLC; Saldana et al 2007). The TLC is a large, narrow nucleus that spans the midbrain tectum longitudinally, very close to the midline and immediately dorsal to the dorsomedial column of the periaqueductal gray matter (PAGdm). This novel nucleus occupies what has traditionally been considered the most medial region of the deep SC and the most medial region of the IC. Scattered throughout the TLC are a few medium-sized neurons, with elongated or triangular cell bodies and more abundant cytoplasm These cytoarchitectural features have enabled the identification of the TLC in a large variety of mammalian species (Saldana et al 2007). TLC neurons respond vigorously to acoustic stimuli (pure tones or wideband noise), but their responses differ from those of auditory responsive neurons in the IC or the SC (Marshall et al 2008)

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Conclusion