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 stim ulation were determined by intracellular recording and biocytin labeling un der in vitro, whole-brain conditions. Based on their axonal projections, la beled neurons (n = 76) were divided into the following groups: TH1 neurons, with mostly ipsilateral projections to the striatum; TH2 neurons, with ips ilateral or bilateral projections to the medial amygdala and nucleus accumb ens; TH3 neurons, with bilateral projections to the medial and dorsal palli um; TH4 neurons, with mostly ipsilateral projections to the striatum and ip silateral 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 t he dorsal thalamus and around the sulcus medialis, and TH5 neurons are foun d in the ventral thalamus. Labeled neurons with ascending projections, i.e. , the more dorsally situated TH1-TII4 neurons, are mostly inhibited by elec trical stimulation of the optic nerve and have significantly longer latenci es (mean +/- S.D., 42.1 +/- 11.6 msec) than neurons with exclusively descen ding projections, i.e., the ventrally located TH5 neurons (8.5 +/- 6.1 msec ), which receive the bulk of retinal afferents and show excitation at elect rical optic nerve stimulation. Neurons recorded without labeling in the dor sal thalamus likewise exhibit mostly inhibition and have significantly long er latencies (35.7 +/- 18.9 msec) than those recorded in the ventral thalam us (10.9 +/- 7.7 msec), which mostly show excitation. None of the neurons r ecorded in the dorsal thalamus followed repetitive stimulation of the optic nerve. Thus, neurons situated in the dorsal thalamus and projecting to pal lial or subpallial telencephalic targets are unlikely to receive monosynapt ic or oligosynaptic, excitatory retinal input. Accordingly, no retino-thala motelencephalic pathway homologous to that found in amniotes appears to exi st in salamanders. (C) 2000 Wiley-Liss, Inc.