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 a s arising from a morphologically homogeneous population of neurons in the stratum opticum (SO). The present study combined immunocytochemist ry with retrograde tracing and lesion techniques to determine whether or not the SC --> LP projection arose from neurons that were also neur ochemically homogeneous. The combination of retrograde tracing and imm unocytochemistry with an antibody directed against calbindin-D 28K (CB D) showed that 64.4% of the neurons that project from SC to LP contain this calcium-binding protein. Retrograde tracing and immunocytochemis try for adenosine deaminase (ADA) showed that a smaller number of tect o-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 dorsomedia l portion of the nucleus and their density was substantially reduced a fter ablation of the ipsilateral superficial SC laminae. The lateral p osterior 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 te cto-LP pathway. Nevertheless, destruction of the ipsilateral SC did re duce the density of CBD-IR in LP. These results suggest that the SC -- > LP projection in rat arises from at least four neurochemically disti nct 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 resul ts further suggest that ADA containing fibers may have a more restrict ed terminal distribution in LP than axons that contain only CBD.