Theta synchronization in the limbic system: the role of Gudden's tegmentalnuclei

Theta rhythm is most prominent in the hippocampus but has also been recorde d in other cortical and limbic structures and can play an important role in functional coupling of widely separated structures responsible for differe nt components of the memory building process. Here we demonstrate in the ra t that neuronal activity exhibiting strong state-dependent synchrony with r hythmic hippocampal electroencephalogram is present also at the brainstem l evel, specifically in the relatively small tegmental nuclei of Gudden intim ately connected with the limbic forebrain. We found that during theta state s, either occurring spontaneously or triggered by sensory stimulation in th e urethane anaesthetized rat, all neurons in the anterior and ventral tegme ntal nuclei exhibited a consistent switch from irregular discharges to rhyt hmic bursts. The switch between these patterns closely matched the analogou s transformations in the hippocampal EEG, but the level of synchrony betwee n the two signals varied depending on the level of theta activation. During sensory stimulation, when theta is faster and more regular, the rhythmic b ursts in the tegmentum showed extremely high coherence (up to 0.96) with hi ppocampal field potentials. During spontaneous theta, the average coherence was lower but still highly significant (0.62). Gudden's nuclei are recipro cally connected to the mammillary body complex (MB) occupying a strategic p osition at the gateway of hippocampofugal connections organized in the Pape z circuit. Thus, coupling between the MB-Gudden circuit and the hippocampus and consequently the neuronal traffic through the Papez circuit and hence the assembly of limbic structures connected to the hippocampus may vary acc ording to the activity in these specific brainstem nuclei.