The medial supramammillary nucleus, spatial learning and the frequency of hippocampal theta activity

Previous studies have shown that the presence of hippocampal theta activity (theta) is important for learning and that the medial supramammillary nucleus (SuM) is involved in the control of the frequency of theta. In the present experiments, a single-day version (20 trials) of the Morris water maze was used to investigate the effects of drug injections into SuM on hippocampal theta frequency and spatial learning. Two groups of rats received an injection of chlordiazepoxide (CDP, 0.5 microl, 40 microg/microl) or saline (0.5 microl) into SuM 10 min before training in the Morris water maze. Two other groups of rats received an i.p. injection of 5 mg/kg CDP or saline, and two further groups received short (10 min) or long (15 min) immersion in cool water (22 degrees C) before training. The results showed: (1) in all groups theta frequency was an inverse logarithmic function of training time; (2) systemic CDP or long cool water exposure decreased theta frequency to a greater extent (by 1 Hz), and also impaired learning to a greater extent, than the other treatments; (3) that SuM-CDP produced a modest decrease in theta frequency (0.35-0.5 Hz) and a modest impairment of spatial learning. These data suggest that theta frequency per se may be important for spatial learning and that total abolition of theta is not necessary for dysfunction; and that while a lesser part of the effect of i.p. CDP on spatial learning appears to be mediated by SuM the greater part appears to involve other nuclei as well.