Structure-function relationships in rat brain stem subnucleus interpolaris. VIII. Cortical inputs.

1. Spinal trigeminal (SpV) subnucleus interpolaris (SpVi) receives inputs from trigeminal (V) first- and second-order neurons, monoamine-containing brain stem nuclei, and somatosensory cortex. Prior studies suggest that SpVi receptive-field (RF) properties cannot be predicted solely on the basis of primary afferent inputs. To assess the cortico-V projection and its role in SpVi RFs, anatomic and electrophysiological experiments were conducted. 2. Phaseolus vulgaris leucoagglutinin (PHA-L) or wheat-germ-agglutinized horseradish peroxidase (WGA-HRP) were used as anterograde tracers to study cortico-V axons in 24 normal adult rats. Injections into SI barrel cortex-labeled pyramidal fibers that decussated at all levels of the V brain stem complex, though crossing fibers were most numerous in the pyramidal decussation and pons. A small number of axons projected to ipsilateral V brain stem subnuclei. PHA-L-labeled pyramidal fibers did not give rise to collaterals in their descent through the pons and medulla. 3. Heaviest terminal labeling occurred contralaterally and in the maxillary portion of caudalis laminae III-V. Moderately dense reaction product was seen in ventral portions of all other contralateral V brain stem subnuclei, as well as in laminae I and II of caudalis. Subnucleus oralis contained the least amount of label contralateral to the injection site. Ipsilateral projections were weak and most dense in principalis. 4. Cortico-V projections were topographic between matching whisker representations. Axons most commonly had longitudinal orientations and stringy shapes. Terminal boutons occurred at the ends of short collateral branches. Many of these collaterals were derived from axons that ascended through caudal V brian stem subnuclei after crossing in the lower medulla. 5. Cortico-V labeling was heavier in septal regions between single whisker representations. This "honeycomb-like" termination pattern was most pronounced in contralateral caudalis and SpVi and ipsilateral principalis. 6. In 13 other adult rats, right SI cortex was aspirated followed by single-unit recordings in left SpVi under pentobarbital sodium anesthesia. In 9 of these, chronic effects were evaluated by recording the responses of 346 left SpVi cells 4-55 days after the lesion. In the remaining four rats, acute effects were analyzed by recording the responses of 190 SpVi cells on the day of the lesion.(ABSTRACT TRUNCATED AT 400 WORDS)