EVIDENCE FOR A GLUTAMATERGIC PATHWAY FROM THE GUINEA-PIG AUDITORY-CORTEX TO THE INFERIOR COLLICULUS

We attempt to provide evidence that the projection from the guinea pig auditory cortex (AC) to the inferior colliculus (IC) may contain glut amatergic or GABAergic fibers. Seven days after unilateral AC aspirati on, histological studies indicated almost complete AC destruction and preterminal degeneration of fibers and terminal fields in the dorsal c ortex (DCIC), external cortex (ECIC), and central nucleus (CNIC) of th e IC ipsilateral to the ablated AC. Contralaterally, degeneration appe ared in the DCIC. AC ablation depressed the electrically evoked Ca2+-d ependent release of D-[H-3]aspartate (D-[H-3]ASP) in the ipsilateral D CIC, ECIC, and CNIC, and D-[3H]Asp uptake in the CNIC, Together with o ther evidence that the corticocollicular pathway is excitatory, these findings suggest that this projection may contain glutamatergic and/or aspartatergic (Glu/Asp-ergic) fibers. Glutamic acid decarboxylase imm unoreactivity was not apparent in presumed pyramidal cells of layer V of the AC retrogradely labeled with biotinylaled dextran injected into the ipsilateral IC. Thus, corticocollicular neurons probably do not s ynthesize GABA and may not be GABAergic. However, AC ablation depresse d [C-14]GABA release from the ipsilateral DCIC and ECIC, and [C-14]GAB A uptake in the DCIC. These findings are consistent with the atrophy o r down-regulation of some subcortical neurons that mediate GABAergic t ransmission in the IC.