EXCITATORY AMINO-ACIDS AS NEUROTRANSMITTERS OF CORTICAL AND CEREBELLAR PROJECTIONS TO THE RED NUCLEUS - AN IMMUNOCYTOCHEMICAL STUDY IN THE GUINEA-PIG

We combined a retrograde labeling technique with peroxidase immunocyto chemistry to verify whether cortical and cerebellar neurons projecting to the red nucleus (RN) contain high concentrations of glutamate and aspartate as possible neurotransmitters. Injections of a tracer, collo idal gold-labeled enzymatically inactive horseradish peroxidase conjug ated to wheatgerm agglutinin, into the RN of adult guinea pigs produce d retrograde labeling of layer V cortical neurons, with a large predom inance in the ipsilateral hemisphere. Corticorubral neurons were locat ed in the granular parietal cortex (Gr), agranular frontal cortex (Ag) , agranular cingulate cortex (Cg), and retrobulbar cortex (Rb). Large numbers of retrogradely labeled neurons were concentrated in contralat eral interpositus and dentate cerebellar nuclei. We found the majority of corticorubral neurons to be immune stained by antibodies raised in rabbits against glutamate or aspartate conjugated to invertebrate hem ocyanin by glutaraldehyde, supporting the hypothesis that excitatory a mino acids are neurotransmitters of corticorubral projections. With ei ther antiserum, immunostaining was found in 58-72% of corticorubral ne urons in Ag and Gr; higher percentages were observed in Rb (80-85%) an d Cg (up to 96%). Cross-sectional area measurements indicated that the perikarya of corticorubral neurons were larger in Ag and Gr than in R b and Cg; in each area, soma size values of immunopositive corticorubr al neurons tended to be larger than those of immunonegative ones. In t he cerebellar nuclei, virtually all retrogradely labeled neurons were immunostained by glutamate and aspartate antisera, suggesting that exc itatory amino acids might also be considered as possible neurotransmit ters for cerebellorubral projections.