GLUTAMATERGIC HIPPOCAMPAL-FORMATION PROJECTIONS TO PREFRONTAL CORTEX IN THE RAT ARE REGULATED BY GABAERGIC INHIBITION AND SHOW CONVERGENCE WITH GLUTAMATERGIC PROJECTIONS FROM THE LIMBIC THALAMUS

Anatomic and physiologic studies in the rat have shown projections fro m the hippocampal formation (HF) and mediodorsal (MD) thalamic nucleus to the medial prefrontal cortex (mPFC). The authors used multi-barrel iontophoresis to: confirm the neurotransmitter used in the projec tio n from HF to mPFC; investigate the role of GABAergic inhibition in the regulation of this projection; and examine the functional convergence of projections from HF and MD onto single mPFC neurons. During HF sti mulation, nine cells (6%) showed excitation followed by prolonged inhi bition, 39 cells (26%) showed prolonged inhibition alone and 100 cells (68%) showed no clear response. In a further 12 cells that showed no predrug excitation to HF stimulation (representing 16% of the cells in this category), iontophoresis of the GABA(A) antagonist bicuculline m ethiodide (BMI) revealed excitatory responses. A total of six mPFC cel ls (38% of the cells showing excitatory responses to HF stimulation) s howed convergent excitation to HF and MD thalamic (or adjacent paraten ial nucleus) stimulation. Five out of eight (63%) of the predrug or BM I-revealed excitatory responses of mPFC neurons to HF stimulation were selectively decreased after AMPA antagonist iontophoresis (either CNQ X or DNQX). These data confirm that the HF projection to prefrontal co rtex is, at least in part, glutamatergic; suggest that the responses o f mPFC neurons to activity in this HF pathway are regulated by GABAerg ic inhibition; and indicate that projections from HF and MD converge o nto single mPFC neurons. (C) 1994 Wiley-Liss, Inc.