Morphology, axonal projection pattern, and responses to optic nerve stimulation of thalamic neurons in the...

Intracellular recording and biocytin labeling were carried out in the fire-bellied toad Bombina orientalis to study the morphology and axonal projections of thalamic (TH) neurons and their responses to electrical optic nerve stimulation. Labeled neurons (n = 142) were divided into the following groups: TH1 neurons projecting to the dorsal striatum; TH2 neurons projecting to the amygdala, nucleus accumbens, and septal nuclei; TH3 neurons projecting to the medial or dorsal pallium; TH4 neurons with projections ascending to the dorsal striatum or ventral striatum/amygdala and descending to the optic tectum, tegmentum, and rostral medulla oblongata; TH5 neurons with projections to the tegmentum, rostral medulla oblongata, prectectum, or tectum; and TH6 neurons projecting to the hypothalamus. TH1 neurons are found in the central, TH2 neurons in the anterior and central, TH3 neurons in the anterior dorsal nucleus, and TH4 and TH5 neurons in the posterior dorsal or ventral nucleus. Neurons with descending projections arborize in restricted parts of retinal afferents; neurons with ascending projections do not substantially arborize within retinal afferents. At electrical optic nerve stimulation, neurons in the ventral thalamus respond with excitation at latencies of 10.8 msec; one-third of them follow repetitive stimulation and possibly are monosynaptically driven. Neurons in the dorsal thalamus respond mostly with inhibition at latencies of 42.3 msec and are polysynaptically driven. This corroborates the view that neurons in the dorsal thalamus projecting to the telencephalon receive no substantial direct retinal input and that the thalamopallial pathway of amphibians is not homologous to the mammalian retinogeniculocortical pathway.