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

In the salamander Plethodon jordani, the morphology and axonal projections of thalamic (TH) neurons and their responses to electrical optic nerve stimulation were determined by intracellular recording and biocytin labeling under in vitro, whole-brain conditions. Based on their axonal projections, labeled neurons (n = 76) were divided into the following groups: TH1 neurons, with mostly ipsilateral projections to the striatum; TH2 neurons, with ipsilateral or bilateral projections to the medial amygdala and nucleus accumbens; TH3 neurons, with bilateral projections to the medial and dorsal pallium; TH4 neurons, with mostly ipsilateral projections to the striatum and ipsilateral projections to the tectum opticum, tegmentum, and rostral medulla oblongata; and TH5 neurons, with ipsilateral projections to the tegmentum, medulla oblongata, and rostral spinal cord without (TH5.1) or with (TH5.2) additional projections to the optic tectum. TH1-TH4 neurons are found in the dorsal thalamus and around the sulcus medialis, and TH5 neurons are found in the ventral thalamus. Labeled neurons with ascending projections, i.e., the more dorsally situated TH1-TH4 neurons, are mostly inhibited by electrical stimulation of the optic nerve and have significantly longer latencies (mean +/- S.D., 42.1 +/- 11.6 msec) than neurons with exclusively descending projections, i.e., the ventrally located TH5 neurons (8.5 +/- 6.1 msec), which receive the bulk of retinal afferents and show excitation at electrical optic nerve stimulation. Neurons recorded without labeling in the dorsal thalamus likewise exhibit mostly inhibition and have significantly longer latencies (35.7 +/- 18.9 msec) than those recorded in the ventral thalamus (10.9 +/- 7.7 msec), which mostly show excitation. None of the neurons recorded in the dorsal thalamus followed repetitive stimulation of the optic nerve. Thus, neurons situated in the dorsal thalamus and projecting to pallial or subpallial telencephalic targets are unlikely to receive monosynaptic or oligosynaptic, excitatory retinal input. Accordingly, no retino-thalamo-telencephalic pathway homologous to that found in amniotes appears to exist in salamanders.