Projections of the nucleus lentiformis mesencephali in pigeons (Columba livia): a comparison of the morphology and distribution of neurons with different efferent projections.

The avian nucleus lentiformis mesencephali (LM) is a visual structure involved in the optokinetic response. The LM consists of several morphologically distinct cell types. In the present study we sought to determine if different cell types had differential projections. Using retrograde tracers, we examined the morphology and distribution of LM neurons projecting to the vestibulocerebellum (VbC), inferior olive (IO), dorsal thalamus, nucleus of the basal optic root (nBOR), and midline mesencephalon. From injections into the latter two structures, small LM cells were labeled. More were localized to the lateral LM as opposed to medial LM. From injections into the dorsal thalamus, small neurons were found throughout LM. From injections into the VbC, large multipolar cells were found throughout LM. From injections into IO, a strip of medium-sized fusiform neurons along the border of the medial and lateral subnuclei was labeled. To investigate if neurons project to multiple targets we used fluorescent retrograde tracers. After injections into IO and VbC, double-labeled neurons were not observed in LM. Likewise, after injections into nBOR and IO, double-labeled neurons were not observed. Finally, we processed sections through LM for glutamic acid decarboxylase (GAD). Small neurons, mostly in the lateral LM, were labeled, suggesting that projections from LM to nBOR and midline mesencephalon are GABAergic. We conclude that two efferents of LM, VbC and IO, receive input from morphologically distinct neurons: large multipolar and medium-sized fusiform neurons, respectively. The dorsal thalamus, nBOR, and midline mesencephalon receive input from small neurons, some of which are likely GABAergic.