Dorsal funicular projections to the dorsal column nucleus in a reptile, Caiman crocodilus

A second auditory area in the non-cortical telencephalon of a reptile

Injections of horseradish peroxidase (HRP) into a caudocentral portion of the non-cortical telencephalon of Caiman known as the dorsolateral area (dorsal ventricular ridge) resulted in retrogradely labeled neurons throughout the entire extent of the ipsilateral nucleus reuniens. HRP-positive cells were most numerous in nucleus reuniens pars diffusa with only sparse labeling of neurons in nucleus reuniens pars centralis. The results of the present experiment, when compared with those of a prior study that determined the telencephalic connections of nucleus reuniens pars centralis, suggested that these two forebrain areas are separate. Staining with succinate dehydrogenase and acetylcholinesterase revealed that nucleus reuniens pars centralis and pars diffusa and their respective telencephalic projection areas can be differentiated on the basis of histochemical features. These findings in Caiman suggest that certain thalamic and telencephalic auditory areas in birds and crocodilians are most likely the result of common ancestry rather than examples of parallel evolution.

Percentage of intrinsic and relay cells in a thalamic nucleus projecting to general cortex in reptiles, Caiman crocodilus

Small injections of horseradish peroxidase (HRP) restricted solely to the general cortex in reptiles, Caiman crocodilus, resulted in retrogradely labeled cells located bilaterally in the nucleus dorsolateralis anterior of the thalamus. Larger cortical injections of HRP labeled greater numbers of nucleus dorsolateralis anterior cells. HRP injections designed to label as much cortex as possible resulted in very few unlabeled ipsilateral neurons in the nucleus dorsolateralis anterior. The percentage of these unlabeled cells was less than 1%. These data indicate that the nucleus dorsolateralis anterior contains few, if any, intrinsic or local circuit neurons.

Percentage of relay and intrinsic neurons in two sensory thalamic nuclei projecting to the non-cortical telencephalon in reptiles Caiman crocodilus

Large injections of horseradish peroxidase were placed in the non-cortical telencephalon in a reptile, Caiman crocodilus. The number of retrogradely labeled and unlabeled neurons in two sensory thalamic nuclei, nucleus rotundus (vision) and nucleus reuniens pars centralis (audition) were counted. The percentage of unlabeled cells in each nucleus was less than 1%. These data suggest that each of these thalamic nuclei contain few, if any, intrinsic or local circuit neurons.