SII-projecting neurons in the rat thalamus: a single- and double-retrograde-tracing study.

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Experiments were performed on adult albino rats, using single-labeling (free horseradish peroxidase [HRP] or wheatgerm agglutinin conjugated to HRP [WGA:HRP]) and double-labeling (fluorescent dyes) techniques to investigate the thalamic projections to the secondary somatosensory cortex (SII) and to demonstrate the presence and location of thalamic neurons projecting to both the primary somatosensory cortex (SI) and SII by way of branching axons. In single-labeling experiments, the tracer was injected in SI or SII with or without electrophysiological control; in double-labeling experiments, fast blue and diamidino yellow were injected into the electrophysiologically identified forelimb areas of SI and SII. Single-tracer experiments showed that after injections in SI, focused in the forelimb representation area, retrogradely labeled neurons were present mainly in the ventral third of the nucleus ventralis posterolateralis (VPL) and in the anterior part of the posterior nuclear complex (PO); labeled neurons were also present consistently in the caudal portion of PO. Injection of tracers in the forelimb or forelimb and hindlimb representation areas of SII resulted in labeling of neurons in the posterior part of PO and in the caudal part of VPL. Double-labeling experiments confirmed the distribution of neurons projecting to SI or to SII, as observed in single-labeling experiments. Some neurons labeled with both tracers were also present. These neurons are interpreted as projecting to both SI and SII by means of axon collaterals and were observed in areas of overlap of the two single-labeled population of neurons--that is, at the border between PO and the ventroposterior complex, and in the medial part of caudal PO. Comparison of these data with those obtained after injections of tracers in SI and SII of cats (Spreafico et al., 1981b) suggests that in both species thalamic neurons projecting to these two areas are largely segregated, though partially overlapping; and that thalamic neurons projecting simultaneously to SI and SII, modest in number in cats, are even sparser in rats.