The sites of termination of the primate spinothalamic tract have been reinvestigated using the anterograde transport of wheatgerm agglutinin conjugated to horseradish peroxidase. Monkeys which received an injection of the conjugate at the spinal cervical level (C7-C8) displayed a "patchy" pattern of labelling in the coronal plane in the ventral posterior lateral and caudal ventrolateral nucleus. In three dimensional reconstructions this labelling appeared to be rod-like in shape. A more homogeneous pattern of labelling was present in parts of the central lateral, posterior, suprageniculate, limitans, submedius, medial dorsal, paracentral, central medial, reuniens and periventricular nucleus. Lumbar injections (L2-L3) produced a similar although less intense pattern of labelling with only the ventral posterior lateral and ventrolateral nuclei displaying an obvious topological organization. Comparison of these results with previous physiological and pharmacological reports suggests several morphological-functional correlations: first, that both the discriminative and motivational/arousal aspects of spinothalamic tract function, associated with the lateral and medial thalamic nuclei, respectively, may be conveyed by direct spinothalamic tract projections. In support of this hypothesis medial spinothalamic tract termination sites receive a homogeneous input which does not have an obvious topographical organization, whereas lateral spinothalamic tract termination sites receive a "patchy" pattern of terminals which are topographically organized; second, that the patchy pattern of labelling observed in the coronal plane in the lateral thalamus corresponds to a "rodlike" pattern of labelling in three dimensions. This "rodlike" pattern of labelling has previously been observed for medial lemniscal projections to the thalamus and has been postulated to be the thalamic equivalent of cortical "columns"; third, that there appears to be a tight overlap between spinothalamic tract terminals and opiate receptor binding in some medial but not lateral thalamic nuclei. Such an overlap may be indicative of a pharmacological difference in the types of spinothalamic tract inputs which could be modulated by opiates at the thalamic level.
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