Ventral tegmental nucleus of Gudden: a pontine hippocampal theta generator?

It is well-established that rhythmically bursting (RB) activity in the medial septum is crucial for the generation of the hippocampal theta rhythm, but the contribution of other diencephalic-pontine structures is less documented. The ventral tegmental nucleus (VTn) of Gudden is related to the Papez's circuit via its interconnections with the medial mammillary nucleus, and therefore it may play a role in the generation of hippocampal theta. In the present study, extracellular activity from VTn neurons were recorded in unanesthetized restrained rats (n = 9). Hippocampal activity (EEG) and electromyograms were recorded simultaneously to identify sleep-waking states. RB activity was observed in VTn during wakefulness, with periods of hippocampal theta and during rapid eye movement (REM) sleep. Rhythmicity in VTn preceded theta activity in hippocampus. The frequency of RB neurons in VTn was 5.6 Hz during wakefulness and 6.8 Hz during REM sleep. It was similar to that of hippocampal theta. The rhythmicity was particularly stable and the firing rates were strikingly high during REM sleep. RB activity in VTn was also recorded from urethane-anesthetized rates (n = 3). Rhythmic firing (4.0 Hz) was slower than in unanesthetized rats and matched the urethane-related theta frequency. Our results show that neurons in VTn exhibit a marked RB activity during states of vigilance accompanied by hippocampal theta rhythm. They suggest that VTn may be a pontine hippocampal theta generator.