Hippocampal θ field activity and θ-ON/θ-OFF cell discharges are controlled by an ascending hypothalamo-septal pathway

Abstract

The nature of the control of hippocampal formation field activity [theta (θ) and large-amplitude irregular activity (LIA)] and θ-on/θ-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 θ in the hippocampal formation, with a linear positive relation between stimulus intensity, θ frequency, and θ 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 θ, 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 m A 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 9 exhibited a gradual recovery with a steeper slope between 20 and 60 min post-procaine. During the time of maximal MS/vDBB suppression, θ-on cell discharges were reduced to 0 in most cases. The initial recovery of θ-on cells in the post-procaine condition was correlated with the first appearance of θ and was characterized by low discharge rates. The gradual recovery to pre-procaine discharge levels (along with discharge pattern) paralleled the recovery of θ amplitude and power, not the recovery of θ frequency. In contrast to the suppression of θ-on cells during the maximal suppression of the MS/vDBB, θ-off cells continued to discharge at rates that were not significantly different from the pre-procaine levels accompanying LIA. The inhibition of θ-off cells normally produced by hypothalamic stimulation was also abolished during the time of maximal suppression of the MS/vDBB. The hypothesis that hippocampal formation θ-on cells received excitatory cholinergic projections and θ-off cells received inhibitory GABAergic projections was supported. Copyright © 1991 Society for Neuroscience.