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Journal of Neuroscience, Vol 11, 2241-2248, Copyright © 1991 by Society for Neuroscience

ARTICLE

Hippocampal theta field activity and theta-on/theta-off cell discharges are controlled by an ascending hypothalamo-septal pathway

JW Smythe, BR Cristie, LV Colom, VH Lawson and BH Bland
Department of Psychology, University of Calgary, Alberta, Canada.

The nature of the control of hippocampal formation field activity [theta (theta) and large-amplitude irregular activity (LIA)] and theta- on/theta-off cell discharges by an ascending hypothalamo-septal pathway was investigated in urethane-anesthetized rats. Electrical stimulation of the dorsomedial-posterior hypothalamus in the range of 0.1-1.0 mA in 0.1-mA steps produced theta in the hippocampal formation, with a linear positive relation between stimulus intensity, theta frequency, and theta amplitude. Reversible blockade of the medial septal (MS)/vertical limb of the diagonal band of Broca (vDBB) region by microinjection of procaine hydrochloride abolished spontaneous and hypothalamically elicited theta, resulting in a field activity at 1-min post-procaine that had a lower power than pre-procaine LIA levels. The efficacy and recovery of the MS/vDBB suppression was tested at 1, 10, 20, 30, 40, 50, and 60 min post-procaine using 0.5, 0.8, and 1.0 mA of hypothalamic stimulation. All three of the dependent measures of hippocampal field activity (frequency, amplitude, and power) showed a progressive recovery during the 60-min post-procaine period. Frequency exhibited a rapid recovery with a shallow slope between 20 and 60 min post- procaine. In contrast, the amplitude and power of theta exhibited a gradual recovery with a steeper slope between 20 and 60 min post- procaine. During the time of maximal MS/vDBB suppression, theta-on cell discharges were reduced to 0 in most cases. The initial recovery of theta-on cells in the post-procaine condition was correlated with the first appearance of theta and was characterized by low discharge rates.(ABSTRACT TRUNCATED AT 250 WORDS)

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