The distribution and morphology of identified thalamocortical projection neurons and glial cells with reference to the question of interneurons in the ventrolateral nucleus of the rat thalamus.

The distribution and morphology of thalamocortical projection neurons and glial cells in the ventrolateral nucleus of the rat thalamus have been investigated using light and electron microscopic techniques. In this material thalamocortical projection neurons in the ventrolateral nucleus were identified by the retrograde transport of horseradish peroxidase following the placement of multiple injections of horseradish peroxidase in the primary motor and sensorimotor overlap regions of the cerebral cortex. The location of horseradish peroxidase-labelled thalamocortical projection neurons varied with the locus of injection in the motor and sensorimotor overlap cortex; caudal injections labelled cells in the rostrolateral region of the ventrolateral nucleus while injections involving successively more rostral regions of the cortex labelled cells in more medial and caudal regions of the nucleus. Labelled thalamocortical neurons were grouped in clusters with neuron pairs often closely applied to each other via non-synaptic junctional complexes. Where the field of labelled neurons was analysed in series of 1-micron-thick sections with phase contrast light microscopy, all neurons in the field were found to be clearly labelled with horseradish peroxidase. Thalamocortical neurons comprised mainly medium-to-large, multipolar ovoid-shaped cells which showed a large centrally placed nucleus with deep invaginations of the nuclear membrane. Two types of glial cells were identified: astrocytes, identified by their characteristic pale-staining ovoid nucleus, showed a close relationship to blood vessels, synaptic complexes and neurons; oligodendrocytes, distinguished by their darkly stained nucleus and cytoplasm, were somewhat smaller and showed a close association with myelinated fibres. These findings show that the rat ventrolateral nucleus comprises a homogeneous population of thalamocortical projection neurons and thus provide indirect evidence suggesting the absence of interneurons in the ventrolateral nucleus of the rat thalamus.