The laminar distribution of theta (theta) field potentials in the entorhinal cortex (EC) was investigated in paralysed and locally anesthetized rats injected with physostigmine in order to induce theta rhythm. Electrode penetrations through the medial, intermediate and lateral subdivisions of the EC showed in all cases: 1. the presence of theta rhythm from layer VI to layer III approximately in phase with CA1 theta rhythm; 2. an amplitude minimum between the outer third of layer III and the inner half of layer I; and 3. a phase-reversed theta rhythm in layers II-I with an amplitude maximum in the outer half of layer I. Results indicate the existence of neuronal sources of theta rhythm in the EC.

Dorsal hippocampal theta rhythm (theta) and extracellular unit activity from CA1 pyramidal layer were recorded in awake guinea pigs, both during standing and during walking on a conveyor belt at increasing speeds. Amplitude, frequency and rhythmicity of theta increased linearly with the movement speed. In this preparation we found the same three types of unit discharge patterns that have been described in anesthetized rats in the presence of spontaneous or induced hippocampal theta: type 1, rhythmic at theta frequency and phase-locked with theta; type 2, discharging non-rhythmically but phase-locked with theta; and type 3, discharging at random. Furthermore, all units modified their firing pattern when the animals walked, either by increasing their rhythmicity and/or phase-locking with theta or by increasing their firing frequency. During walking, some type 3 units changed into type 2 or type 1, type 2 units changed to type 1, and type I increased their rhythmicity. Consequently, the unit discharge rhythmicity and phase-locking with theta increased with the speed of movement. The mean rate of neuronal discharges increased linearly as a function of walking speed. In this paper we show that the progressive spectral theta changes determined by the intensity of movement are concomitant with the increase in rhythmicity of hippocampal cells. Moreover, the firing rate of these cells, and the amplitude, frequency and rhythmicity of theta, increased linearly as a function of walking speed, suggesting that neuronal excitation may be basically responsible for these changes in theta properties.

The activity of 72 neurons recorded in the reticularis pontis oralis nucleus (RPO) was examined in anesthetized and curarized rats during hippocampal theta (theta) rhythm elicited by either sensory stimulation or carbachol microinjections. During hippocampal theta rhythm evoked by sensory stimulation, 63.9% of RPO neurons increased their discharge rate while the firing rate decreased in 20.8%. In all cases, the RPO neurons maintained a non-rhythmic discharge pattern. In 44% of the neurons the discharges tended to occur on the positive wave of the theta rhythm. Similar firing patterns were seen in 18 RPO neurons recorded during theta rhythm elicited by both, sensory stimulation and a carbachol microinjection; this effect was blocked by atropine. These results indicate that the RPO region contributes to the generation of hippocampal theta rhythm with a tonic and nonrhythmic outflow through a cholinergic system which may be muscarinic.

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