Reformulation of the Papez circuit: absence of hippocampal influence on cingulate cortex unit activity in the primate

Cingulate cortex (CC) unit responses to hippocampal volleys and afterdischarges were studied in 4 awake unsedated squirrel monkeys using extracellular microelectrodes. Stimuli were delivered via bipolar electrodes chronically implanted bilaterally in anterior and posterior hippocampus. A total of 412 units in 21 tracks were studied and tested to single and multiple shock stimulation. Spontaneous firing rates were approximately equally distributed among slow (0.25 spikes/s), medium (2.5-10), and fast (10-25). Only 2% of cingulate units (9/412) responded orthodromically to hippocampal stimulation: 5 with initial excitation; 4 with initial inhibition. Eight responses had initial latencies from 100-200 ms; one unit a latency of 25 ms. Three units displayed periodic, almost cyclical, excitation. All unit response thresholds were 1 shock, 0.25 mA or less. Hippocampal afterdischarge activity influenced the firing pattern of 17 units: 16 showed intraictal excitation and 1 inhibition. In comparison with the preceding studies in the same model these results indicate that the influence of hippocampal stimulation on cingulate cortex is not nearly as prominent as either the fornix or non-fornix hippocampal influence on hypothalamic, preoptic, and basal forebrain structures. It is proposed that the cingulate cortex is not a prominent link in the Papez circuit.