Immunochemical heterogeneity in the tecto-LP pathway of the rat

The projection from the rat's superior colliculus (SC) to the lateral posterior nucleus of the thalamus (LP) has previously been described as arising from a morphologically homogeneous population of neurons in the stratum opticum (SO). The present study combined immunocytochemistry with retrograde tracing and lesion techniques to determine whether or not the SC-->LP projection arose from neurons that were also neurochemically homogeneous. The combination of retrograde tracing and immunocytochemistry with an antibody directed against calbindin-D 28K (CBD) showed that 64.4% of the neurons that project from SC to LP contain this calcium-binding protein. Retrograde tracing and immunocytochemistry for adenosine deaminase (ADA) showed that a smaller number of tecto-LP cells (15.7%) were immunoreactive (IR) for this enzyme. Moreover, nearly all (93.0%) of the ADA-IR tecto-LP cells also contained CBD-IR. Adenosine deaminase-IR axons in LP were restricted to the dorsomedial portion of the nucleus and their density was substantially reduced after ablation of the ipsilateral superficial SC laminae. The lateral posterior nucleus contained numerous CBD-IR cells and fibers throughout its extent and it was thus difficult to determine the extent to which the extra-perikaryal CBD-IR in this nucleus was dependent upon the tecto-LP pathway. Nevertheless, destruction of the ipsilateral SC did reduce the density of CBD-IR in LP. These results suggest that the SC-->LP projection in rat arises from at least four neurochemically distinct cell groups: 1) those that contain CBD, 2) those that contain both CBD and ADA, 3) a very small population that contains only ADA, and 4) a group that is not recognized by either of these markers. Our results further suggest that ADA containing fibers may have a more restricted terminal distribution in LP than axons that contain only CBD.