GLUTAMATE-DECARBOXYLASE IMMUNOREACTIVITY IN CORTICOCORTICAL PROJECTING NEURONS OF RAT SOMATIC SENSORY CORTEX

Combined retrograde tracing and immunocytochemical experiments were ca rried out on rats to ascertain whether corticocortical projecting neur ons in the somatic sensory areas are immunoreactive to an antiserum ag ainst glutamate decarboxylase. Injections of a retrograde tracer (coll oidal gold-labelled wheat germ agglutinin conjugated to enzymatically inactive horseradish peroxidase) in the first somatic sensory area lab elled neurons in the injected area, in the second somatic sensory area , and in the parietoventral area of the ipsilateral hemisphere. The to pographical and laminar distribution of these retrogradely-labelled co rticocortical neurons in the first and second somatic sensory areas an d in the parietoventral area was in line with a previous description ( Fabri M. and Burton H. (1991b) J. comp. Neurol. 311, 405-424). In sect ions processed for the simultaneous visualization of the retrograde tr acer and glutamate decarboxylase immunoreactivity, a number of neurons were double-labelled. Double-labelled neurons were most numerous in t he first somatic sensory cortex, where they accounted for 5% of ail re trogradely-labelled neurons. Outside this region, double-labelled cell s were observed in the second somatic sensory cortex and in the pariet oventral cortex, where they amounted respectively to 2.8% and 2.3% of all corticocortical neurons labelled in these two areas. Glutamate dec arboxylase-immunopositive corticocortical neurons were mainly concentr ated in the infragranular layers (73.8% of all double-labelled neurons in the first somatic sensory area, 81.7% in the second somatic sensor y area, and 76.5% in the parietoventral area). The results indicate th e presence of a small but significant contingent of GABAergic inhibito ry neurons in the associative connections of the somatic sensory areas .