Functional connections of the rat medial cortex and basal forebrain: an in vivo intracellular study

Projections between the medial cortex and basal forebrain in the rat were demonstrated by intracellular recordings and the anterograde tracer Phaseolus vulgaris leucoagglutinin. Direct projections between these areas were indicated by antidromic action potentials, short latency (less than 5 ms) orthodromic potentials, and labeled axon terminals in the basal forebrain subsequent to iontophoresis of Phaseolus vulgaris leucoagglutinin into posterior cingulate cortex. High proportions of antidromic action potentials were encountered in responsive cortical neurons (66%) and basal forebrain neurons (97%). Antidromic latencies recorded in the basal forebrain (less than 1.0 ms) revealed fast ascending projections; cortical neurons showed both fast and slow descending projections (latencies of 0.3-3.7 ms). Relatively few synaptic potentials (none in the diagonal band of Broca) and sparse labeling of axon terminals observed in the basal forebrain indicated that the ascending projections may be the more physiologically important or, at least, densest pathway. Polysynaptic feedforward pathways were suggested through long latency (greater than 20 ms) inhibitory and excitatory postsynaptic potentials, the former being the more common response. Candidate inhibitory neurons were identified in both cortex and basal forebrain. Possible monosynaptic (less than 5 ms) inhibitory postsynaptic and antidromic responses in these cells provided evidence that candidate inhibitory neurons participate in the reciprocal pathways.